A skull says warning. Crossed bones say danger, but in a way people now recognize as playful when it appears on costumes and decor. Crossed swords add action. A map suggests a quest. A key suggests a locked secret. A compass suggests the wearer is going somewhere, even if that place is only the snack table.
That is the useful thing about pirate symbols: they are small but efficient. A plain black shirt becomes pirate-coded with the right mark. A simple costume becomes clearer with a sash, buckle, bandanna, or flag. A shelf becomes a little scene with a ship, map, coin, or skull tucked into the arrangement.
The best displays do not use every symbol at once. Choose two or three and let them cooperate. A flag plus a map feels like a voyage. A hat plus a hook feels like a character. A ship plus a small flag feels like a whole harbor in miniature.
Symbols are shortcuts. Pirates, naturally, approve of shortcuts.