I did a Pirate Theme several years ago. I built a ship's deck onto the front porch of my house. I created an 8x8 deck frame (built to code with 16" on center joists) and covered it with treated outdoor plywood planks. For the front, I just cut it into a triangle for the bow of the ship. On the sides, I used outdoor siding plywood (With a wood texture) and some 2x2 posts for support and created the sides of the ship. I added a mast made of 6" PVC pipe and a ship's wheel from a wooden ship's wheel I had and a wheelhouse I created from 2x8s. Here are some pictures:
Two of us built this 24' long pirate ship for our Lion's Club Haunted Hayride. It was built in three 8' long sections that each had to be trailered out to the field, assembled and brought back in each night. We used traditional 2x4 construction and luan plywood to get the curved effect. PVC drainpipe was used for the masts.
One year, we eliminate the middle section, bolted the two trailers together and used it as a parade float, since it was finished and painted on all sides.
I did Pirates last year, and built half a ship made from cardboard. Had to modify it for Halloween as the weather in the UK was terrible so had to put it up in my living room. I found cardboard shaped better, it just depends how big you wanted it really. Here is a pic of my indoor boat from last year