Map from Google - Samoa

At Samoa (formerly Western Samoa), archipelago in the Pacific Ocean located south of the Equator, the climate is tropical, that is hot, humid and rainy throughout the year with a maximum in the rainfall amount from December to March, and a relative minimum from June to September.
The temperatures are stable and vary little between the hottest period (December to April), when highs are about 30 °C (86 °F), and the coolest period (June to September) when they are around 29 °C (84 °F). The temperatures at night are almost always above 20 °C (68 °F). The trade winds blow steadily throughout the year, making the heat more bearable.
Here are the average temperatures of Apia.
Average temperatures - Apia
ApiaJanFebMarAprMayJunJulAugSepOctNovDec
ApiaJFMAMJJASOND
Min (°C)242424242424232323242424
Max (°C)303030303029292929303030
Min (°F)757575757575737373757575
Max (°F)868686868684848484868686

The rains fall in the form of downpour or thunderstorm, intense but usually not long-lasting, except during the period from December to March, when they can sometimes last for several hours.
Rainfall is enhanced by the mountains that are found in inland areas of the two main islands, and in particular the largest island, Savai'i, where Mount Silisili, 1,858 metres (6,096 ft) high, is located. Here the rains exceed even 5 metres (195 inches) per year.
In the capital Apia, in Upolu island, 2,800 millimetres (110 inches) of rain per year fall, with a maximum of 450 mm (17.5 in) in January, and a minimum of 80 mm (3.2 in) in July and August. Here is the average precipitation.
Average precipitation - Apia
ApiaJanFebMarAprMayJunJulAugSepOctNovDecYear
ApiaJFMAMJJASONDY
Prec. (mm)45038035025016012080801301702603702800
Prec.(in)17.71513.89.86.34.73.13.15.16.710.214.6110.2
Days1918171513118912141617169

The Samoa Islands are exposed to tropical cyclones, which develop in the South Pacific from November to mid-May, although they are more likely from December to March.

Samoa

For swimming, the sea is warm all year round, as we can see in the following table.
Sea temperature - Apia
ApiaJanFebMarAprMayJunJulAugSepOctNovDec
ApiaJFMAMJJASOND
Sea (°C)292929292928282828282929
Sea (°F)848484848482828282828484

When to go

The best time to travel to Samoa goes from June to September, being the least warm and rainy; however, even in this period some downpour or thunderstorm must be put into account.

What to pack

All year round, tropics-friendly, loose fitting clothing, sun hat, a scarf for the breeze, a light sweatshirt for the evening, a light raincoat or umbrella. For the mountains, hiking boots, fleece, raincoat.