Whale watching in Tonga

Hi, I am planning on travelling around Tonga for a couple of weeks in July 2008.
My main objective is to swim with the whales. Am I better off booking a whale watching tour before I leave or is it easy to book once in Tonga? Do they book out.
Thanks for your advice

Whale watching is best done in Vavau or Haapai. It is easy enough to book once you get there. In Vavau there are a couple of operators so you should be able to find one that has a few spots left - we never had a problem making a booking (they aren't always full).