Items? You can't buy leather armor as both a pirate and a non-pirate? Swords, bows, daggers, crossbows, maces, etc. aren't a part of both? You can't have sea encounters? I suppose you DO have to have a boat to be pirates, where you wouldn't on the land, but the PHB assumes boats as part of the game, so there is still no change to the base assumption of the game by playing pirates.
I'm going to refer you to the 5e DMG where it talks about sea adventures and encounters, which means that it is assumed as part of the game. Nothing changes except location. Being pirates is no different to the base assumption of the game than being a land traveling party.