I belong to an active F150 forum where towing is a hot topic. The consensus seems to be that while it's not a problem for boat trailers to have a tongue weight of 10-15% (the commonly cited figure, esp. for travel trailers), it's generally OK for boat trailers to have less tongue weight, in the 6-8% range. This is due to the fact that boats have less frontal and side areas that are susceptible to wind, and the shape of the boat makes it more aerodynamic, which helps it track more stably. If your boat trailer isn't squirrelly at highway speeds and tracks well behind your tow vehicle even in windy conditions, I would think you're OK.
Nothing wrong with shooting for 10-15%, of course, but if the trailer's pulling fine at 8-9%, I wouldn't lose any sleep over it.
