Hi Mate
I went to Tonga a few years Back for a Business trip (Ha'api island) and did about 10-12 dives while there - this was OFF season so no whales - great weather nice beach very very clear water and warm , very expensive also
I am a fairly experienced Instructor and dived all over the world bit spoiled probably , but i thought Tonga had some of the most boring Diving i have ever done in the world , out of all the diving there were very very very few fish (1 school of barracuda) and apart from that not much else. House reef at 2-3 meters deep was OK i pottered around with camera for couple hours ,
But everywhere i dived had almost no fish and limited corals (clear water)
So my answer is i would not bother if no whales - but if you are there anyway i would do a few dives for fun - also depends on your diving History and experience if you only done 5-20 dives before you might think it's great but for me without whales was just EMPTY - nice rock formations but no fish.
All i can say is I am glad i didnt pay full price for trip and diving as knew someone there.
Other people might have better ideas about Vavu (more famous for diving) but for me there are way better places to go for diving.
and way cheaper - all indonesia, malaysia, maldives etc , etc
Hope that helps
Mark