Map from Google - French_Polynesia

In French Polynesia, a vast archipelago in the South Pacific Ocean, the climate is tropical, influenced by the trade winds, with a hot and humid season from November to April, and a cooler season from May to October. The hot season is also the rainiest, besides, from December to April, there's the risk of cyclones (except in the Marquesas Islands). The temperatures vary little throughout the year, they are high in the northern islands, and gradually cooler as you move the south.

In the westernmost area of French Polynesia, we find the group of the Society Islands, divided between the Windward Islands in the west (Tahiti, Moorea, Tetiaroa, Mehetia, Maiao), and the Leeward Islands in the east (Bora Bora, Huahine, Maupiti, Raiatea, Tahaa, Manuae, Maupihaa, Motu One, Tupai). In these islands, the temperatures vary little throughout the year, just by a few degrees; the daytime temperatures are about 30 °C from December to April, and they go down to around 27/28 °C from June to August.
The amount of sunshine is fairly uniform throughout the year, which means that even in the rainiest months, there's no shortage of sunshine: simply, the showers are more intense and more frequent, but they won't last long. In the larger islands, which have highlands in the interior, like Tahiti, the slopes facing east and therefore exposed to the trade winds, are rainier than the western slopes, however, the islands are completely covered by forest, which means that there are not a real dry season anywhere.
Papeete, the capital, is located on the north-west coast of the island of Tahiti, and receives 1,800 millimetres of rain per year, with a maximum in January, and a minimum in August, however, rainfall exceeds 150 mm per month from November to April. Here is the average precipitation.
Average precipitation - Papeete
Prec. (mm)33029016017012080604050801603001840

Further to the south, on the west coast, Punaauia and Paea are slightly more sheltered and sunnier. In the island of Tahiti, in the sultriest and hottest months, during the night a breeze blow from the mountains, called Hupe, reducing a little the feeling of sultriness.
Here are the average temperatures of Papeete.
Average temperatures - Papeete
Min (°C)232424232322212121222323
Max (°C)303030302928272728282828

The sea temperature is high throughout the year in the Society Islands, as you can see in the following table, concerning the water temperature in Tahiti.
Sea temperature - Papeete
Sea (°C)282829292827272626272728

In the flatter, but also located more to the north, Bora-Bora atoll, about 2,000 mm of rain per year fall, and the rain pattern is similar, although the rains a bit more frequent and abundant throughout the year; however, even here there's a minimum in August, of 70 mm of rain. The temperature is slightly higher than in Papeete, due to the northern position.
Average temperatures - Bora Bora
Bora BoraJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)242424242422222222232424
Max (°C)303131313029292829293030

The Society Islands can be hit by tropical cyclones from November to April (though less frequently than the westernmost islands of Oceania), as it has been the case with Oli, which hit Tahiti and Bora Bora in February 2010, or Hosea, that struck Bora Bora in late November 1997, a year of El Niño.

Bora Bora

The best time to visit the Society Islands goes from May to mid-October, or better, from June to September, being the coolest and the least rainy.

To the east, the Tuamotu Islands have a climate similar to that of the Society Islands, just a little less rainy, especially in the eastern part, in fact they receive around 1,200 mm in the eastern part, and 1,500 mm in the western part; also because they are flat atolls, without mountains which could enhance the condensation of moisture. The least rainy period runs generally from May to September. Here is the average precipitation in the north-western island of Takaroa.
Average precipitation - Takaroa
Prec. (mm)260145125135901007070801301551801540

Furthermore, these islands are less exposed to tropical cyclones, since they are located in the eastern edge of the area where they can form.
Here are the average temperatures of Hao.
Average temperatures - Hao
Min (°C)252525252423232323232424
Max (°C)303030302928272727282829

Even the temperature of the sea at the Tuamotu Islands is similar to that of the Society Islands, as can be seen in the following table.
Sea temperature - Hao
Sea (°C)282828282827262626262727

In the southern part of the Tuamotu, there is the atoll of Mururoa, now uninhabited. South of the Tuamotus lie the Gambier Islands, which are associated with them administratively, but they have a slightly different climate, because they are located more to the south, therefore we will discuss them later.

The Marquesas are the northernmost islands of French Polynesia, and therefore have a warmer climate, but above all are almost always out of the path of tropical cyclones. This means that in theory they can be visited all year round, although they are definitely muggy from December to April. Here are the average temperatures in Atuona.
Average temperatures - Atuona
Min (°C)232324242323222222222323
Max (°C)313131313029292929303131

Here the rains have a slightly different pattern: they are not very abundant, being below 1,500 mm per year, and they have a peak in June and a minimum in October and Novembre; however, the rains are quite erratic from year to year. Being equipped with hills in inland areas, the Marquesas have different microclimates depending on slope exposure, so that some coasts are almost dry. However, considering also the rainfall records, the best time here runs from August to November. Here is the average precipitation.
Average precipitation - Atuona
Prec. (mm)90851151009519011590906555751165

The sea in the Marquesas is always warm, due to the location near the Equator, so that the water temperature never drops below 27 °C.
Sea temperature - Atuona
Sea (°C)282829292828282727272828

In Atuona, in the island of Hiva Oa, the painter Paul Gauguin, which spent here the last years of his life, is buried, even though he had previously lived for years in Tahiti.

Gauguin, Nave Nave Moe

The Austral Islands are divided into the groups of Tubuai (Tubuai, Rimatara, Rurutu, Raivavae, Îles Maria), which is located around the Tropic, and group of the Bass Islands (Rapa Iti, Marotiri), which lie more to the south; for their southern location, it's useful to include in this group also the Gambier Islands, which are the southernmost part of the Tuamotu-Gambier administrative group, south of the Tuamotu, of which we have already spoken.
These islands are cooler throughout the year: in the Tubuai and the Gambier the average temperature goes from 25 °C in January to 20/21 °C in August; this means that the period from December to April is warm but not excessively, while the period from June to September, when the daytime temperatures are around 24 °C, can be a bit cool for beach life.
Average temperatures - Tubuai
Min (°C)232323222019181818192022
Max (°C)282829282624242424252627

Besides, in these islands the rains are frequent and relatively abundant even in the period from June to September, because they can be reached in winter by the western disturbances. For example, in Rikitea, in September, the driest month, still 120 mm of rain fall. Objectively, you cannot find a better period for these islands. Here is the average precipitation in Tubuai.
Average precipitation - Tubuai
Prec. (mm)215220230190135901351701451101402252005

Even the sea temperature in the Tubuai Islands decreases in the winter months, although it remains acceptable for swimming, since the water temperature drops to 23 °C from July to October, as can be seen in the following table.
Sea temperature - Tubuai
Sea (°C)262727262524232323232425

The Bass Islands are located even more to the south, and south of the Tropic: here it's rare to see hot temperatures, since the daytime temperature goes on average from 25 °C in the period January-March, to 19/20 °C in the period from July to October. Here the rains are abundant throughout the year, and the total amount is above 2,500 mm per year, in addition the wind is often intense. These islands do not lend themselves to beach tourism.
Average temperatures - Ahurei
Min (°C)222222201917161616171820
Max (°C)252525232221201920202123

When to go

The best time to visit the Society Islands (Tahiti, Bora-Bora) and the Tuamotus runs from June to September, while for the Marquesas Islands it runs from August to November.
For the Austral Islands there is not a more advisable season, because the hot and muggy summer of the Southern Hemisphere, with some risk of heavy rains and thunderstorms and also tropical cyclones, is followed by the austral winter which is a little cool, and even rainy and windy in the southernmost islands.

What to pack

For the Marquesas, Society and Tuamotu Islands, all year round, tropics-friendly, loose fitting clothing, sun hat, a scarf for the breeze, a light sweatshirt for the evening, light raincoat or umbrella. For the reef, equipment for snorkeling, water shoes or rubber soled shoes.

For the Austral Islands, from June to September (ie in winter), light clothes for the day, a scarf for the breeze, a sweatshirt for the evening, possibly a light jacket, light raincoat or umbrella.

For the Bass Islands, from June to September (ie in winter), spring/autumn clothes, sweatshirt or sweater, jacket, raincoat or umbrella; from December to February, light clothing, a sweatshirt for the evening, light raincoat or umbrella.