The Solomon Islands are hot and humid year-round, averaging around 27°C (81°F). There are two distinct seasons: a wet season from November to April and a drier season from May to October.
The best time to visit is May to October. The south-east trade winds bring cooler, drier weather, seas are calmer, and visibility for diving is at its best. June through August is peak tourist season — not that it's ever crowded — so book accommodation and domestic flights ahead if you're travelling then.
The wet season from November to April brings heavy rain, especially January through March, and occasional tropical cyclones. The Solomons sit at the north-western edge of the South Pacific cyclone zone, so they're less frequently struck than places like Vanuatu, but it does happen. Diving is still possible in the wet, and prices are lower, but expect some days lost to weather.
Water temperature stays around 28-29°C year-round — no wetsuit needed, though a 3mm shorty is handy on longer dives.