Each winter July-October, over a hundred humpback whales come to Vava'u to bear their young before returning to the colder Antarctic waters for the southern summer. They generally stay on the western side of the group, in the lee of the prevailing trade winds. Lots of tour and fishing boats around Vava'u do whale-watching trips, and you'll easily see eight whales on a good day. Ask if there will be a roof on your boat, as the sun can be merciless.
To maximize profits, the Tongan government requires all whale-watching operators to charge a minimum fare and you'll need to book early as demand often outstrips supply.
This is one of the few places in the world where you can actually get in the water and swim with the whales. Some operators don't always observe standard practices regarding human/whale encounters and to satisfy clients will motor directly toward whales, get too close, and drop snorkelers directly in the path of the whales.
Whale Watch Vava'u based on Mounu Island uses a specially designed boat with a hydrophone that allows the mating songs of the males to be broadcast over speakers on board.
WhaleSwim Adventures has been operating at Vava'u since 1999. They offer upscale four, seven, and nine-day whale watching trips around Vava'u and can make all your arrangements at premium prices.
Whales in the Wild based at Hakula Lodge also organizes seven-day whale watching packages on hydrophone-equipped boats.