Map from Google - Guadeloupe

In Guadeloupe, the climate is tropical, hot and humid all year round, with little variation in temperature between the coolest months, which are January and February, with about 24 °C as daily average, and the warmest months, from June to October, with an average of 27 °C. The most sweltering months are September and October.
Here are the average temperatures of Basse-Terre.
Average temperatures - Basse-Terre
Basse-TerreJanFebMarAprMayJunJulAugSepOctNovDec
Basse-TerreJFMAMJJASOND
Min (°C)202021222324242323232221
Max (°C)282828293031313131302928

Guadeloupe is an island, overseas region of France, actually formed by two islands, connected with each other: Basse-Terre (the western island, where the regional capital of the same name is located) and Grande-Terre. Besides them, there are also some smaller islands: Marie-Galante, Iles des Saintes and La Désirade.
Rainfall in Guadeloupe is abundant: on the coast, more than 1,800 millimetres of rain fall annually, of which more than 100 mm per month from April to December, and more than 200 mm from July to November. The driest month is February, with an average of 60 mm.
Here the average rainfall in Pointe-à-Pitre, in the central part of the island, where the airport is located.
Average precipitation - Pointe-à-Pitre
Pointe-à-PitreJanFebMarAprMayJunJulAugSepOctNovDecYear
Pointe-à-PitreJFMAMJJASONDY
Prec. (mm)9060701151601452051902452452101451880
Days151110111414181718181717180

In inland areas, the island of Basse-Terre is more humid and rainy, because of the volcano Grande Soufrière, 1,467 metres high: along its slopes the sky is often cloudy, the climate is equatorial, that is, humid and rainy all year round, and a rainforest grows.
The rains occur throughout the year as downpours or thunderstorms, which soon leave room for the sun, except when the archipelago is hit by tropical cyclones, called hurricanes in the Atlantic and the Caribbean. Hurricanes may occur from June to November, but they are most likely from August to October, as happens in the Antilles.

Guadeloupe

The sea is warm enough all year round, as we can see from the following table.
Sea temperature - Guadeloupe
GuadeloupeJanFebMarAprMayJunJulAugSepOctNovDec
GuadeloupeJFMAMJJASOND
Sea (°C)272626272728282829292827

When to go

The best time to visit Guadeloupe goes from January to March, being the driest. In December and April, the rains can sometimes be abundant. In the summer months, in June there is a relative lull in precipitation, but it's now the most sultry period, when even hurricanes are possible.

What to pack

All year round: light, tropics-friendly clothing, possibly a light sweatshirt and a light raincoat for thunderstorms; from December to February, a light sweatshirt for the evening.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
A sweatshirt and hiking boots for climbing Grande Soufrière.