Mike: There are several thread here about 20's and even a few 22's on trailers, and the general consencus is that it is not recommended.
the reasons for this include:
1. Most 20" tires are not rated for the weight, and even if they are, they are not specifically made for trailering.
2. Ride. If you use the small sidewall, low profile type of tire, the boat will receive a harsher ride on rough roads
3. Wheel rating. Most wheels would be rated for enough wieght, but check for sure.
Someone had a link to an article about the construction of trailer tires and how they differ from auto tires. There was a lot of difference.
That said, if I were not going to tow very far, it would bother me to use bigger tires. I think Extreme trailers even offers 20" wheels as an option, but I may be wrong, it may just be 18's.