Hey Scubaman, I hope I'm not too late for your reply. I just got back from Tonga last week. We spent one week in Eu'a and one week in Vava'u where most of the whale swimming takes place. The whale swims were incredible and will only get better until october. fantastic encounters with males in heat runs as well as mothers and calves.
The diving is a different issue. We did some diving in Eu'a, namely a site called The Cathedral which is an enormous underwater cave. This is more expensive (for no reason as it is not further from the harbor than anywhere else) but the site is well worth doing for its grandeur and sun shafts poking through the interior. We had about 6 giant tuna patrolling the edges of the massive cave and that was it. Not many other fish.
Diving in Vava'u was disappointing. The coral is wonderfully healthy and we spotted moray eels, sea snakes and a leaf scorpion fish however fish life is not prolific and it is well known that the region has been overfished. The exceptions mentioned above were only spotted by myself and another diver. Overall the diving is not really worth doing unless you just want to be underwater. You're better off spending the money on another whale swim and day out on the boat if you're expecting to see lots of fish life. So despite the good points I mentioned above, Tonga is NOT a scuba diving destination. There were comments by many that it was most surprising that there were no fish to see! Does not hold a candle to ribbon reefs in australia or even fiji where most tourist have stomped out a lot of the coral. The country is amazing and rustic with a beautiful and friendly people who don't ask for money. No sandy beaches and very non-commercialized. Amazing for nature lovers and for those wanting a peaceful holiday. The only complaint i have is that I could not leave my accomodation in Eu'a for 5 meters without seeing a whale breaching ;P
