Pirate Map

Ocean enemies produce it, but only ones the game counts as worth money, which rules out anything a statue spawned along with things like Meteor Heads that carry no coins. You also have to be out in the far stretch of the world and close to the surface. Farming from the bottom of a shaft below the ocean floor stops the drop dead, and that is the usual reason a setup runs for an hour and produces nothing.

Get above the open water with Water Walking Boots or a platform, put a Water Candle down, drink a Battle Potion, and let minions handle the spawns. You stay in range of everything while most of it has no footing to reach you from. A Blood Moon on top of that turns the trickle into a flood. A small graveyard near the coast keeps daytime spawns high, so you are not restricted to nights.

King Slime is a good repeatable source if you would rather summon than wait, and the airborne spawns count too, so Harpies and Wyverns over the water are as good as anything swimming in it. For real volume, reshape the coast. A contained patch of Crimson beside the ocean brings Blood Jellies in faster than the ocean spawns on its own, jungle grown near the water brings Arapaimas, and in a Corruption world spreading the ocean with Unholy Water or a Clentaminator lets you grind the Eater of Worlds, whose many segments each roll for the drop. Neptune's Shell lets you do all of this standing on the sea floor.

Pirates fought over the ocean drop more maps, and so do the enemies of any other invasion that shows up while you are standing there. A Goblin Army arriving mid-farm is a bonus rather than an interruption, and fighting one invasion in the right place is how you bank the next.

At least one player needs 200 maximum health before the game will accept it, so bank a few Life Crystals first. The map only drops in Hardmode, but it works anywhere: carry a stack to a fresh world and a pre-Hardmode character can be wearing pirate gear long before the invasion would normally come knocking.