Map from Google - Wallis_And_Futuna

The climate of Wallis and Futuna, an archipelago located in the Pacific Ocean south of the Equator and belonging to France, is equatorial, hot and humid throughout the year, with a warmer and more humid period from November to April, and a slightly cooler period from May to October. However, the difference is little: during the day, the temperature goes from 30/31 °C (86/88 °F) to 28 °C (82 °F) in the Futuna Islands, and from 30/31 °C (86/88 °F) to 29 °C (84 °F) in the northern island of Wallis, which is closer to the Equator. However, in the period from May to October, and especially from June to September, the south-east trade winds blow constantly, cooling the air.
Here are the average temperatures of Futuna.
Average temperatures - Futuna
FutunaJanFebMarAprMayJunJulAugSepOctNovDec
FutunaJFMAMJJASOND
Min (°C)252525252524242424242425
Max (°C)303130302929282829293030
Min (°F)777777777775757575757577
Max (°F)868886868484828284848686

For swimming, the sea is warm all year round, as we can see in the following table (in the island of Wallis the sea temperature is also slightly higher).
Sea temperature - Futuna
FutunaJanFebMarAprMayJunJulAugSepOctNovDec
FutunaJFMAMJJASOND
Sea (°C)292929292828272727282829
Sea (°F)848484848282818181828284

The rains are plentiful, as they exceed 3,000 millimetres (118 inches) per year; the wettest period is the warmest, when more than 300 mm (12 in) per month fall: this happens from October to April in Wallis and from December to April in Futuna, which, being more distant from the Equator, has a shorter peak in the rainfall. However, in both islands it rains all year round: precipitation never drops below 100 mm (4 in) per month, although between a shower and the other there is plenty of room for sunshine.
Here is the average precipitation in Futuna.
Average precipitation - Futuna
FutunaJanFebMarAprMayJunJulAugSepOctNovDecYear
FutunaJFMAMJJASONDY
Prec. (mm)3653703753102551401851852152352753803295
Prec.(in)14.414.614.812.2105.57.37.38.59.310.815129.7
Days211921181612131413161619198

The amount of sunshine is acceptable, because the rains, though frequent and sometimes heavy, are usually short-lived.

Wallis and Futuna is exposed to tropical cyclones from November to mid-May, though they are more likely from December to March.

Wallis

When to go

The best time to visit Wallis and Futuna runs from June to August, being the least hot and the least rainy; however, even in this period showers and thunderstorms are fairly frequent.

What to pack

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.