Sailor Hat

The rank and file of a Pirate Invasion carry it. Corsairs, deckhands, crossbowers and deadeyes can all cough up a piece, while the captain and the heavier units cannot, so a fast clear that skips straight to the boss is the wrong way to farm it. Invasions turn up on their own once a world is in Hardmode, and if you would rather not wait on the message, a Pirate Map from the ocean enemies starts one on demand.

Every part goes in a social slot and takes nothing away from your real loadout, so there is no cost to running it over your armor. You already know the look from the fight itself: the hat is what the corsairs wear and the shirt comes off the deckhands, so the full outfit with the pants puts you in the uniform of the invasion you just cleared.