1. the frame has to hold it.
2. the drive train has to pull it.
3. the brakes have to stop it.
When a car maker specifies a towing weight for a passenger car (as opposed to a vehicle designed for towing, like a pickup), the limiting factor is usually the brakes.
Seriously, don't exceed the capacity in your owner's manual, in fact, try to stay under 70% of it -- it *will* push you out into an intersection, or into that a**hole who just cut in front of you on the freeway (and he *will* cut in front of you, believe me).
Trailer length is also a factor -- a longer trailer can take control of a short-wheelbase vehicle.
Of course, the strain on the transmission (not to mention clutch) will seriously shorten its useful life (heed the "external transmission cooler" advice above).
You asked "would this work" for 14 to 20 foot boat -- my opinion, for a 14' flatbottom jonboat with a 6-horse motor, sure. But 16, 18, 19, 20-footer...please, please don't do it. There are people on the road whose families depend on 'em, and killing another motorist is one sure way to ruin an otherwise perfectly good fishing trip.