No way dealers make 5%....they couldn't pay their bills if they did. Nobody would stay in business with those razor thin margins. Dealers make money on the boat itself(I'm guessing 10-15%), manufacturers incentives, financing, and service.
As for service...I doubt that the dealerships make much on "under warranty" work, tho. I would imagine that they make quite a bit of money on financing...esp since the finance terms tend to be quite long(10 years).
Add-ons...and the labor required to install them....are most likely the highest profit margin area for dealers.