Map from Google - Solomon_Islands

The Solomon Islands, located in the Pacific Ocean south of the equator and east of Papua New Guinea, have an equatorial climate, hot, humid and rainy all year round, so much so that they are covered by rainforests.
Temperatures are stable throughout the year: highs are 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, prevail.
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, in which there are inland elevations that protect it from the trade winds (see below).
Average temperatures - Honiara
HoniaraJanFebMarAprMayJunJulAugSepOctNovDec
HoniaraJFMAMJJASOND
Min (°C)232322222221212121222222
Max (°C)303030303030303030303030
Min (°F)737372727270707070727272
Max (°F)868686868686868686868686

Rainfall is decisively 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 around 200/250 mm (8/10 in) per month. In the islands which have some elevations in the interior, 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 such as Mount Popomanaseu, 2,335 metres (7,661 feet) 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
HoniaraJanFebMarAprMayJunJulAugSepOctNovDecYear
HoniaraJFMAMJJASONDY
Prec. (mm)2803003302151401001151151151501652352255
Prec.(in)1111.8138.55.53.94.54.54.55.96.59.388.8
Days202022181715171516161619211

Another mountainous island is Kolombangara, where lies Mount Veve, 1,768 metres (5,801 feet) 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 are able to produce rainclouds.
The amount of sunshine in the Solomons is never great, nor poor; it's fairly uniform throughout the year, with some 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 tropical cyclones of the South Pacific 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 (in the group of the 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
HoniaraJanFebMarAprMayJunJulAugSepOctNovDec
HoniaraJFMAMJJASOND
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 they may even be abundant in the south-facing slopes, especially in the mountainous islands. In July, the rains often increase compared with the other months of this period, except in the northern slopes of mountainous islands such as 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.