Drowned Farm (Tridents & Copper)

Drowned are basically the underwater version of zombies, spawning in ocean water at night or anywhere dark enough underwater. A drowned farm is a zombie-farm layout adapted for flooded rooms instead of dry ones. Same idea, wetter execution.

Building over open ocean rather than near a coastline tends to give better rates, for the same reason a creeper farm benefits from being over water: fewer land mobs nearby competing for the shared spawn cap.

You need a roof to keep the room dark even in daylight, since drowned can spawn underwater at any hour as long as it's dark enough down there, unlike land zombies which burn in direct sun. Trapdoors create the same slow funneling current used in a skeleton farm, sweeping drowned toward a drop point.

The real reason to build this specific farm, rather than just fighting drowned when you happen across them, is the trident. Drowned are the only mob that can spawn already holding one, and a farm running long enough will eventually hand you several, which otherwise can't be crafted at all. Tridents matter for Riptide and Loyalty builds, and separately make fighting the Elder Guardian a lot more comfortable.

Copper and nautilus shells (for conduits) come along as a steady bonus. Since this farm needs no rare structure like a spawner or a monument, you can build it almost anywhere there's open ocean once you've got basic materials and a water source.