Mike,
The trailer gross weight cannot exceed the kerb weight of the car. So in your example, a car weighing 200kg, could tow a 2 ton boat and trailer - this is a rough guide and would need checking in the manufacturers handbook.
750kg is the upper limit for an un-braked trailer, after this brakes must be fitted and be correctly working by law. (Believe me, you wouldnt want them not working on anything over 1 ton, unless you have one of the huge 4x4's)
Re. towing a cat B type boat. Not practical on a regular basis. OK once or twice a year i.e when the boat comes out for a scrub and clean, but I feel that regular towing and launching ends at 20' ish.
Hope this helps?
Adam