Bruce and all,
We come down during all times of the year (except over Christmas - it's too busy). I like the summer the best for the same reasons you stated - fewer people, less crowded, etc. (in fact we are on Provo now and will be until mid-August). But I haven't found a time of the year (other than Christmas) that is bad. Yes high season (mid-November to mid-March or so) is busier, but the people get spread out. If you go around Point Grace to the east of the Sands even in January, the beach is practically deserted in front of the Point Grace and Mansions complexes. We own a condo here (Ocean Club West - same management company running the place since they opened Ocean Club East in 1993 when I first started coming here) and spent 5 months here last year and will spend about 4 months here this year. So far this year we have been here from mid-January to mid-February (whale watching season), the month of May, mid-July to mid-August (our current visit) and will be back for the month of November (nothing like Thanksgiving under the palm trees!). Already booked our mid-January trip for next year.
With airline cutbacks, there are fewer seats available to fly into Provo and so there will be low vacancy rates at most places which means fewer crowds most of the year on all the beaches and boats. I wouldn't hesitate to come at any time of the year, other than Christmas and New Year's. Even in November, we have rarely had a problem getting into any restaurant even without a reservation. Some of the smaller dive operators (especially Flamingo) do book up early (by the way their max is eight divers, I see in my earlier post my eight turned into a smiley face). But most of the larger operators, like Caicos Adventures, Provo Turtle and Dive Provo, usually can accommodate another diver or two on short notice.
I have been diving with all the operators on the island and i generally go with Ocean Vibes (small operation and the only natively owned operation - plus Wayne is a great guide/captain/person). Flamingo is another favorite because of the small groups (plus I have known Mickey and Jane for many years). If I go with a larger operator, I generally go with Provo Turtle Divers primarily because I have got to know Art Pickering quite well when he owned it and have gotten to know the new owners since they took over in 2003 or 2004.
Hope that additional insight is helpful.