I did some diving over at Tulagi in April this year.
The water temp is pretty much good all yea round, we had 30 C while we were there. Viz was down a little for us as there was a weeks solid rain before we arrived. In saying that we still had ~ 20m for most dives.
If you are interested in wrecks Tulagi is the premier wreck destination in this part of the world. The are just so many wreck options. The highlight being the USS Aaron Ward. In saying that the reefs are fantastic too, lots life and diversity.
I used Tulaghi Dive (
Tulagi Dive, Solomon Islands ) when I was over there I highly recommend them. They can cater for diving levels all the way up to trimix diving and have a great setup. (i think you might have to give them some notice to make sure they have the He if u are after trimix).
I plan to go back next year, the diving is that good!