Map from Google - Tokelau

Tokelau, an archipelago consisting of three coral atolls, is a territory of New Zealand in the South Pacific Ocean, and has a hot, humid and rainy climate throughout the year.
The temperature is stable: in the northernmost atoll of Atafu the daily average is around 82.5 °F throughout the year, and goes down to 80.5 °F in the southern atolls, therefore further away from the equator, of Nukunonu and Fakaofo.
Here are the average temperatures of Nukunonu.
Average temperatures - Nukunonu
NukunonuJanFebMarAprMayJunJulAugSepOctNovDec
NukunonuJFMAMJJASOND
Min (°F)757575757575757575757575
Max (°F)868686868686868686868686

The rainfall is abundant, amounting to around 110 inches per year, with a rainier period between November and March, when precipitation is not less than 10 inches per month, and a relatively less rainy period between April and September, when however it doesn't go below 6 inches per month. Here is the average precipitation in Nukunonu.
Average precipitation - Nukunonu
NukunonuJanFebMarAprMayJunJulAugSepOctNovDecYear
NukunonuJFMAMJJASONDY
Prec. (in)14.211.49.67.16.58.17.76.76.97.99.612.2108.1
Days

Tokelau lie in the path of tropical cyclones, which can pass from November to mid-May, although on June 1997 they were affected by the "latecomer" cyclone Keli.

Tokelau

For swimming, the sea is warm all year round, as we can see in the following table.
Sea temperature - Tokelau
TokelauJanFebMarAprMayJunJulAugSepOctNovDec
TokelauJFMAMJJASOND
Sea (°F)868684848484848484848686

When to go

The best time to visit the Tokelaus runs from mid-May to October, being the driest (though showers and thunderstorms are still relatively frequent) and out of the period of cyclones. Given the albeit rare precedents in which cyclones have occurred outside of this period, for greater security, you can restrict the best period to July-mid-October.

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.