Map from Google - Guam

In Guam, an island administered by the United States, which is located in the western Pacific Ocean and geographically is part of Micronesia, the climate is tropical, hot and humid all year round, with temperatures around 30/32 °C (86/90 °F) during the day, and 24/25 °C (75/77 °F) at night.
Here are the average temperatures of Tamuning.
Average temperatures - Tamuning
TamuningJanFebMarAprMayJunJulAugSepOctNovDec
TamuningJFMAMJJASOND
Min (°C)242424252525252525252525
Max (°C)303030313131313131313030
Min (°F)757575777777777777777777
Max (°F)868686888888888888888686

The rainfall is abundant, above 2,000 millimetres (79 inches) per year; the rainiest period is from July to November, the driest is from January to April, in which, however, there's no shortage of showers and thunderstorms. Here is the average precipitation.
Average precipitation - Tamuning
TamuningJanFebMarAprMayJunJulAugSepOctNovDecYear
TamuningJFMAMJJASONDY
Prec. (mm)1159065851251602653653403102151402275
Prec.(in)4.53.52.63.34.96.310.414.413.412.28.55.589.6
Days191617171922252524252322254

The amount of sunshine in Guam is never excellent, even though it is acceptable or good from January to June; on the other hand it's never poor, because even in the wettest months, between the strong showers, the sun can peep out.
The island lies in the path of typhoons, which can cause more or less serious damage, bringing rain and strong winds, and occur from mid-May to mid-November, although sometimes they have passed over the island outside of this period, as was the case with Roy (January 1988), Isa (April 1997), Kong-rey (not strong, April 2007), and Pongsona (December 2002).

Guam

For swimming, the sea is warm enough all year round, as can be seen in the following table.
Sea temperature - Guam
GuamJanFebMarAprMayJunJulAugSepOctNovDec
GuamJFMAMJJASOND
Sea (°C)282828282929292929292928
Sea (°F)828282828484848484848482

When to go

The best time to travel to Guam is from February to April, being the driest, and out of the typhoon period.

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.