This past Summer, took our 15 foot fiberglass on a long road trip
instead in the usual half mile to the marina. The tires are the small
4.80-8 utiltiy type tires, and needless to say, they overheated
badly.....
long story, but I'll be upgrading to a biger tire this Summer.

Here's my question. It seemed that the speed at which the load was
being carried created the heat. While pondering the trip home
(semi desperate and after a few beers) I considered filling the tires
with water (say 50%) in an attempt to dissipate the heat to the rims.

I never did this, but have pondered the the effects of water in a tire
at speed. Would the balance go for a bundle or would the water be
thrown evenly within the tire by the certifugal force?

I made the trip home by upping the pressure to 60 p.s.i. and by
driving slower.

Would water have helped of would I have been courting a disaster?