I have few ideas for the boats:
sailor (goes around the ship repairing holes,working the pumps,ect. and should have a dagger)
grapeshot cannon (fires a bunch of small projectiles that pierce the ship and do a ton of damage to crew)
Captain (has a flintlock in his right hand and a sabre in his left hand and should have double the health of a regular unit)
Molotov (a pirate that throws molotovs and burns enemy ships)
drunk pirate (this guy drank a bit to much beer....but he throws bottles at enemys and when the bottles hit something the glass goes and cuts everything around it so he's a bit.....useful?)
platypus (yep...this thing is actually harmful.the male one's anyways.it stabs enemys with the back of its feet poisoning enemys.and there small which makes them a hard target.who knew that such a cute and cuddly creature would be so deadly?)
special units:
Captain Black Beard (has a blunderbuss in his left hand and a sabre in his right hand and should obviously have a black beard and should have a lot of health)
Shark Rider (its a pirate with a blunderbuss on a shark and the shark bites chunks out of a ship and the pirate would shoot the enemys and if the pirate dies then the shark is still alive and if the shark dies then the pirate is still alive....but the pirate would drown or be eaten by other sharks or whatever)
sharkman (this guys a beast......literally.he can swim in water and has a shark head so he bites people and he can board enemy ships.he could take out a ship that has only sailors and little resistance by his self.)
octoman (........why?he is a beast like sharkman and can grab and throw or grab and eat people's heads off.he's actually good friends with sharkman and together they can destroy any ship.exept for very large ships,such as the man-o-war but they would damage it a lot.oh and he has the head and tentacles of a octopus.)
mega platypus (because platypuses weren't enough.this thing is absolutely huge and has lasers shooting out of its eyes.well it doesn't have lasers shooting out of its eyes but its huge.and stings people with the back of their feet poisoning them and the poison does more damage then a regular platypus.)
that's all I got I hope you guys like my suggestions.