I'll throw in my 2 cents.
First of all, I think most of the time the 10 to 15% rule needs to be thrown out when towing boats. Any good boat manufacturer designs their trailer to be a perfect fit to the boat, regardless of the weighting. I have a boat that weights about 6000lbs on the trailer and the hitch weight was 340lbs (I measured it). It tows like a dream.
Secondly, single axle trailers are prone to sway. That is simply a matter of fact and it sucks. A lot of this can be negated because the boat is highly aerodynamic, as opposed to a TT or a car hauler. This aerodynamic effect is another reason why I don't buy the 10 to 15% rule.
Now in your case, it sounds like the boat manufacturer did a bad job matching the trailer to the boat. So that sucks. I think your best plan of action is to get a sway controller designed for a straight hitch and move as much weight forward as you can. (towing with less fuel in it could also help).
http://www.hitchsource.com/equalizer-pole-tongue-adapter-p-27444.html
You don't want the weight distribution, just the sway component. Weight distribution will be the opposite of what you want.
I'd also recommend going to a dedicated shop for this and asking what they would do. I'm sure they see this stuff all the time.