I just feel they can fit way to many crew and w Marines can run around close to a 400 man crew. Also the big open deck loaded w 300 + crew should be more susceptible to causalities from both cannon balls and grape. For gods sake they'd have to be packed on board shoulder to shoulder.
Making the ship/crew a little more fragile and reducing the numbers to 300 would go a long way to balancing things out a little better.
Just my opinion as I'm still a noob at this game and certainly don't have the expertise that you all possess.