Tonga has great diving and it is best done in the winter months (June-Sept) for best visibilty and the chance of whale encounters. Yes the water is a bit nippy - average around 75F, but you warm up fast when a humpback cruises by or sings a tune.

Vava'u has the most established dive industry and lots of operators, the best is Dive Vava'u IMHO. Vava'u has very nice caverns with huge fans and nice hard coral formations

For the best trip allround, I would recommend NAI'A, the Fijian liveaboard that goes to Tonga for the whale season each year - this is the best chance to have great encounters because you are onboard, sunrise to sunset looking for whales for around 9 days (different each year) and finding the right whales is a numbers game.

Most of the whale swim operators in Vava'u and again Dive Vava'u is the one we found to be the most professional and respectful of the whales - you'll find lots of claims about who will get you in the water fastest and most often, but there is a world of difference between seeing whales underwater, and having hour long encounters with whales that have approached you, rather than jumping in briefly with whales that have just stopped running to give the calves a rest.

Swimming with humpbacks is IMHO the single greatest privelage I have had in the water and I can't recommend it highly enough. Get to Tonga and throw some money into whale tourism before the Japanese whalers knock them all off "for research purposes".

Never chase the whales, just wait and hope they come back for another look. You will never get a good shot otherwise.

Check out the promo we made for the Tonga Visitors Bureau here:
http://www.underseaproductions.com/tvb.html

Cheers
Josh