Here is some information for you on the areas where the operators are in.
Tongatapu is the main island, there is currently one operator who is there and is offering tours, however I am not sure whether he does just whale watching or the snorkeling with whales.
Ha'apai is the middle group of islands approx 60 miles North of Tongatapu, Chathams Pacific the domestic airline flies Monday to Saturday I believe once a day. They have a website for booking information. For whale trips in Ha'apai, there are three licenced operators, Ha'apai has low lying sandy atolls which can be subject to weather. During the time the whales are here is our winter, so we have the trade winds which blow an average of 20-22 kts per day, so the more sheltered water an island group offers you then the better the places to hide, which is one of the reasons that Vava'u has the most operators (13 licenced).
You can find out the operators on a couple of different site: http://tongaholiday.com and http://thekingdomoftonga.com. I would suggest writing to different operators.
By law here, operators may only put 4 people plus the guide in the water, there are some operators that are more aware about the whales behaviour, which in my consideration means that you will get better opportunities as the longer you take to get a whale relaxed, the more chance you have of long swims.
Swims are never guaranteed, and there are some days that the whales are traveling and concentrating on other behaviours, as well as obvious weather conditions.
There are also people that run photography trips and you can find out about those through other members here and what is going on.
Hope this helps in someway if I can give you anything more specific please let me know.
Many thanks
Karen