Port at any seaward town and give the crew orders to take prisoners before returning to the ship. Once they have come back, send them out again. (The prisoners will be pressed into service and added amongst the ranks of the sailors.) Repeat as often as necessary.

When battling at sea, attack your opponent's sails with Chain shot. (Without a sail, the ship becomes less mobile.) Once the sails are tattered, switch to Grapeshot and pound the deck. (This will slay most of the crew.) After a few strikes, sail alongside the ship and issue the grapple order to begin boarding.

All you have to do is attack a port. When you do this, the number of marines will either double or triple(try not to keep more than 300 or your food and rum supply will be consumed quickly).

In the game the most common ship is the frigate. The galleon is at least three times the size of a frigate, and there are only two in the game as far as I know.

One is in the port of Havana. Every now and then, the Spanish Armada will be docked there, and this is a fleet of about twelve frigates, twelve pinnaces, and one galleon, all armed to the teeth. If you try to take these guys on the sea, you're usually dead within five minutes.

However, you can exploit this shore leave. When they're in port, go to the battle screen but do not attack the town. Instead just take a ship with a hundred marines and board that really big ship (the galleon). All of the crew of the armada are in the port so every ship will be defenceless from boarding. Do this, and you have the biggest pirate ship in the game.

The second galleon belongs to a pirate and is never in port, but with this first galleon, you may be able to take it down. Good luck!