Pirates are a large part of popular culture these days, and for a skilled Lego builder, it's a snap to put together a ship for any of your piratical needs. There are a large number of sets made by Lego, even a pirate-themed series of toys made by the company. However, with a large enough collection of pieces, you can make your own set that's cheaper and more personal.
A ship only becomes ship-shape when it has a hull. Without a hull in place, there's nothing to build the rest of the ship on, and no way it could ever float. The hull is the large, round wooden part at the bottom of the ship (see the diagrams at Tin Pan in resources if you have any questions), the part which actually floats in the water. If you have any pirate-themed Lego sets, then you likely have pieces to make a hull, or you can design your own from scratch. There are even some Lego sets with waterproof ship hulls, which would allow your pirate ship to really float.
If you are building the hull from scratch, start with flat plates to make up the bottom of the hull. You can make the boat as long as you want to, but it should be longer than it is wide: the largest hulls from Lego are usually about 18-inches by 6-inches, and about 4-inches tall. The front, or prow, of the boat is usually pointed, while the rear--the stern--is often rounded. The design is up to you, as the creator. For more building challenge, try adding a hinge to part of the boat so you can play below deck. Some larger ships also have a lower level of cannons, so be sure to include a row of cannon ports if you want them on the hull.
After assembling the hull, build the main deck across the top of the hull. Most ships have a railing that extends around the edge of the deck. Many also have a captain's cabin, and an upper deck, both at the rear. On the large Lego ships, the Captain's cabin takes up the rear three to four inches of the ship, and the upper deck is on top of that. This deck is also where the wheel is located, as it's near to the rudder--which sticks out the back of the hull, at the center. The deck-mounted cannons should be installed after assembling the deck. For a real authenticity, you should leave a point for the main mast to stick through the center of the deck.
Masts and Rigging
As impressive as your ship is now, it does need some way to move. This is accomplished with a series of sails, at least one large one suspended above deck on a large mast. The mast is a single, long post and usually has what's called a crow's nest at the top: a tiny, round observation deck. Longer ships have room for two or three masts, allowing multiple sails and increased speed. For more ideas on how to lay out your sails, see Tin Pan in resources.
To make sails, you can either use the sails included in most Pirate-themed Lego sets, or make your own set with your own designs on them, using paper. You can also use some paper to make your own pirate flag. While the most common pirate flag is the Jolly Roger design, the well-known skull and crossbones, a number of pirates also had their own. (See resources for more design ideas.)