If you see Pascal in your town, give him a scallop (Not a white scallop. That doesn't work.) and he will give you a random Pirate Ship theme item. Do not talk to him until you have the scallop with you or he swims away.

Theme item	How to unlock
Anchor	Give Scallop
Barrel	Give Scallop
Helm	Give Scallop
Keg	Give Scallop
Sea View Wallpaper	Give Scallop
Ship Cannon	Give Scallop
Ship Compass	Give Scallop
Ship Deck Flooring	Give Scallop