Map from Google - Solomon_Islands

The Solomon Islands have an equatorial climate, hot, humid and rainy throughout the year, so much so that they are covered by rainforest.
The temperatures are stable throughout the year, with highs around 30/31 °C (86/88 °F) from November to April, when the warmer and wetter winds from the north-west predominate, while they drop slightly from May to October, when the south-east trade winds, relatively cooler, blow.
Here are the average temperatures of Honiara, the capital, where the daytime temperatures are stable, since it is located on the north side of an island, which is more sheltered from the trade winds (see below).
Average temperatures - Honiara
Min (°C)232322222221212121222222
Max (°C)303030303030303030303030
Min (°F)737372727270707070727272
Max (°F)868686868686868686868686

Rainfall is abundant, since it ranges from 3,000 to 3,500 millimetres (118 to 138 inches) per year in the flat islands, which are the majority, and it's well distributed throughout the year, even though it reaches a peak from January to March, and decreases slightly from May to November, when, however, it remains at around 200/250 mm (8/10 in) per month. In the islands which have some internal elevations, the rainfall is more abundant in the slopes exposed to the wind and less abundant in the sheltered slopes: in particular in the island of Guadalcanal, there are volcanoes like Mount Popomanaseu, 2,335 metres high, which protect the northern slopes from the trade winds during the cooler season, so that in the capital Honiara, on the north side, less than 150 mm (6 in) of rain fall from May to October; here precipitation amounts to "only" 2,200 mm (87 in) per year, with a maximum in March, which is typically the wettest month in all the islands. Here is the average precipitation in Honiara.
Average precipitation - Honiara
Prec. (mm)2803003302151401001151151151501652352255

Another mountainous island is Kolombangara, where lies Mount Veve, 1,768 metres high: here too there is a difference in the period of the trade winds, between the north side, more sheltered, and the south side, more exposed to the winds which create rainclouds.
The amount of sunshine in the Solomons is never great; it's fairly uniform throughout the year, with many hours of sunshine, but also frequent cloud formation, which may lead to showers or thunderstorms.
The Solomon Islands are located at the edge of the area where the South Pacific tropical cyclones form, from November to mid-May, therefore they are affected to a lesser extent than other areas of the South Pacific. Outside of this period, there has been Cyclone Xavier, which affected the southeastern province of Temotu (Santa Cruz Islands) on late October 2006.

Solomon Islands

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

When to go

The best time to visit the Solomon Islands is from June to October, being the least muggy and rainy, bearing in mind, however, that showers are frequent even in this period, and indeed they may be abundant in the south-facing slopes, especially in the mountainous islands. Often the rains increase in July compared with other months of this period, except the northern slopes of the mountainous islands like Guadalcanal. The north side of Guadalcanal, where Honiara is located, is by far the most sheltered area of all the islands, in the period from June to 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. For Mount Veve and Popomanaseu, spring/autumn clothes, sweatshirt, jacket, raincoat, hiking shoes.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.