Map from Google - Uruguay

In Uruguay the climate is temperate, with hot summers, and quite rainy throughout the year. In most of the country, the average temperatures are similar to those of the Mediterranean coasts, while in the far north they are a little higher. Compared with the Mediterranean climate, however, here even summer is rainy, due to thunderstorms, which, however, do not reduce by much the sunshine hours. In addition, in Uruguay the clashes of air masses are more intense and therefore the temperature variations are higher than in the more sheltered Mediterranean Sea.
Being in the Southern Hemisphere, the seasons in Uruguay are clearly reversed compared with Europe or North America.
The annual precipitation hovers around 1,000/1,100 millimetres in the south (see Montevideo), while it's higher in the north-central, where it's around 1,200 mm, but in the far north it reaches 1,500 mm.
Here is the average precipitation in Montevideo.
Average precipitation - Montevideo
MontevideoJanFebMarAprMayJunJulAugSepOctNovDecYear
MontevideoJFMAMJJASONDY
Prec. (mm)8510010585908585909511090851100
Days88889910881098103

Winter is fairly mild, but it can be windy and unstable, especially in the south. In winter, snowfalls are very rare, even though outbreaks of the cold wind called Pampero can occur throughout the country, with possible slight frosts at night, from May to September.
Summer is hot, the sun often shines, but in the afternoon thunderstorms can break out, more likely in the north. Heat waves are possible, with peaks of 38/40 °C, from December to March in the south-central, and also in November in the far north.
Spring and autumn are mild, but the weather is often unstable.
The amount of sunshine is good from November to March, while in winter, there are periods of bad weather, in which the sky is cloudy, alternating with sunny days.
In the capital, Montevideo, the average temperature goes from 10 °C in July to 23 °C in January. In summer the heat is tempered by breezes (more than in Buenos Aires, located on the other bank of the Rio de la Plata, but more to the interior). As seen in the table, rainfall is fairly constant throughout the year, but with a relative maximum in autumn (March-April) and spring (October-November). Even in other areas of Uruguay, March is often the rainiest month, but the highest records of rainfall have occurred often in the month of April.
Here are the average temperatures of Montevideo.
Average temperatures - Montevideo
MontevideoJanFebMarAprMayJunJulAugSepOctNovDec
MontevideoJFMAMJJASOND
Min (°C)18181613108789121416
Max (°C)282826221915151618212427

In the far north, the climate is warmer: in Artigas the average of June and July doesn't go below 13 °C, while in January it reaches 26 °C.

Montevideo

For swimming, the Atlantic Ocean is a bit cool, but still acceptable, from January to March: at Punta del Este it reaches 22 °C in January and March, and 23 °C in February.
Sea temperature - Punta del Este
Punta del EsteJanFebMarAprMayJunJulAugSepOctNovDec
Punta del EsteJFMAMJJASOND
Sea (°C)222322201714121213151820

When to go

In Uruguay, the sunniest season is summer (which in the north runs from November to March and in the south from December to February), but it can be hot and has the risk of afternoon thunderstorms. Winter is mild, but sometimes it can be unstable, rainy and windy, especially in the south, and sometimes even cold. Therefore, the intermediate seasons are preferable: autumn (March and April in the south, April and May in the north), and spring (October and November in the south, September and October in the north). Autumn, however, is often more rainy, so we prefer spring. If you do not suffer from the heat, you want more sunshine and maybe you want to spend some time at the beach, you will choose summer.
For a beach holiday, summer is clearly the best period, especially January and February.

What to pack

In winter (June to August): clothes for spring and autumn, sweater, warm jacket, raincoat.
In summer (December to March): light clothing, a sweatshirt for the evening, umbrella or light raincoat.