Map from Google - Northern_Mariana_Islands

The climate of the Northern Mariana Islands, U.S. territory in the Pacific Ocean, is tropical, hot and humid throughout the year, with a relatively cool and dry season from December to June, and a warmer and rainy season from July to November.
The temperatures are stable throughout the year, especially in the southern islands (which are the only inhabited ones), where the daytime average goes from 28/29 °C (82/84 °F) in the coolest period, to 30/31 °C (86/88 °F) in the warmest. The heat is sultry but tempered by the trade winds.
Here are the average temperatures of the capital Saipan.
Average temperatures - Saipan
SaipanJanFebMarAprMayJunJulAugSepOctNovDec
SaipanJFMAMJJASOND
Min (°C)242324252526252525252525
Max (°C)282829303031303030303029
Min (°F)757375777779777777777777
Max (°F)828284868688868686868684

Rainfall is abundant, so that it reaches 2,400 millimetres (94.5 inches) per year in the southernmost island (Rota), and 2,000 mm (79 in) in Saipan and Tinian. The wettest months are August, September and October, with more than 250 mm (10 in) per month, while in the least rainy months, from February to May, from 70 to 100 mm (1.6 to 4 in) of rain per month fall.
Here is the average precipitation in Saipan.
Average precipitation - Saipan
SaipanJanFebMarAprMayJunJulAugSepOctNovDecYear
SaipanJFMAMJJASONDY
Prec. (mm)1107580100951252302853252851801202010
Prec.(in)4.333.13.93.74.99.111.212.811.27.14.779.1
Days221919181919212321222122246

The other islands, located north of the three permanently inhabited ones, are gradually cooler in winter, and generally drier throughout the year, although along the windward slopes, exposed to the north-east trade winds (being volcanic islands, with some elevations in the interior), the rains may become locally more abundant.
The amount of sunshine is never excellent, because there's always the chance to see some cloudiness, which can develop and lead to thunderstorms, but it's never bad either, because even in the rainiest months, between showers, the sun comes out.
The Northern Mariana Islands are located in the path of typhoons, the tropical cyclones of south-east Asia, towards which they normally move after passing over the islands. The period in which they typically occur is from mid-May to mid-November, although at times they have passed at other periods, in April (typhoon Isa in 1997, Kong-rey in 2007), December (Pongsona in 2002), and even January (Roy in 1988).

Saipan

The sea temperature is warm enough for swimming all year round: it drops to 27 °C (81 °F) in February and March.
Sea temperature - Saipan
SaipanJanFebMarAprMayJunJulAugSepOctNovDec
SaipanJFMAMJJASOND
Sea (°C)282727282829292929292928
Sea (°F)828181828284848484848482

When to go

The best time to visit the Northern Marianas runs from January to April, being the least warm and rainy, and even outside the typhoon season. February and March are the best months of all.
The sea is warm all year round.

What to pack

All year round, light clothes, of natural fibres, 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.