Leechs are fairly dangerous mobs that roam around the floors of Submerged Swamps. They are dark in colour and quite small, making them difficult to spot.
Leeches will attack a player until they latch on. A player with a Leech latched onto them will periodically take two lots of half a heart of damage in quick succession. That player can beat the Leech in the face (aim for the mouth) with any melee weapon both whilst it's attached and whilst it's roaming about.
It is best to take them out with a ranged weapon if you have a keen enough eye to spot them at a reasonable distance.
Leeches seemingly appear larger after latching and detaching from a player, as they detach after a short time. It is unknown if this is true or if we're all just a little insane.