Cut pieces of corrugated cardboard in varying lengths from 2 to 4 feet long and about 6 or 8 inches wide.
Tack the cardboard pieces to the bed frame so that they run parallel to the bed frame and each other. Allow the cardboard pieces to overlap slightly to look like the wooden hull of a pirate ship. If you want, extend the pieces to the floor by attaching them together with double-sided tape. Place the pieces on the headboard as well.
Paint a large black or blue circle on the cardboard on the headboard to simulate a porthole.
Paint a pirate ship name on the cardboard hull of the boat.
Hang a Jolly Roger flag on a post that already exists on the bed, or attach the flag to a broom handle and secure it with masking tape or string to the headboard or another part of the bed.
Lay a blue rug or sheet on the floor next to the bed to simulate water.
Hang a fish net over the headboard on the wall and place plastic fish or a buoy or two into it.