Leeches

Leeches are a water-based swarming enemy in Rain World. They live solely in water and if they find themselves on land they will attempt to crawl back to nearby water. If they fail to reach water quickly they dry out, wither away, and die. Leeches come in red and blue varieties.

They are not a source of food (dead or alive).

When a living creature (excluding Leviathans) enters the territory of leeches, they will swim after it, even occasionally jumping out of the water. A leech does not deal damage like a bite or explosion, but will instead attach itself to its prey and weigh it down in an attempt to drown the creature. Once enough leeches have latched on, a creature will be unable to ascend poles or swim to the surface for breath, depending on the strength of the creature. If attacked by leeches, it is suggested the player reach dry land as soon as possible and shake them off by jumping back and forth left to right. If you're on a pole, balance on top and rock back and forth to shake them off. Depending on the individual Leech, they will turn black after a long enough time out of water and lose grip on their prey. After a while after completely drowning an entity they will just let the body float about and ignore it like nothing happened you can try that out in the sandbox by placing let's say 15-25 blue leeches in the water and place let's say a Miros and watch it struggle and eventually it will just give up and drown but without a good fight I would recommend doing that in the Waterworks arena so it will still have a chance to survive.

Red Leeches
Red Leeches are the most common variety, a little smaller than Slugcat and found in pools of water in Drainage System and Garbage Wastes.

Sea Leeches
Sea Leeches are a bigger version of the Red Leech. They are harder to dehydrate out of water and are very heavy and fast. The Sea Leeches are found in the room before the gate from Garbage Wastes to Shoreline, and throughout Shoreline.

Strategy
Both variety of Leech target any creature in the water (dead or alive), meaning it is possible to distract them with a pursuing predator or Batfly. You can also use a Snail to knock out the Leeches.

Jetfish, due to their high speeds and ability to jump out of the water, are very effective at dodging Leeches, especially in Shoreline. Squidcadas can be found in the room before the gate from Garbage Wastes to Shoreline, where there is a pool full of Sea Leeches. It is recommended to grab a white one and hop along the tops of the poles sticking out of the water.

If none of these options are available, such is usually the case in Drainage System, and swimming through them is the only available route, then;
 * Watch how they move. A Leech will only latch onto a player or creature during its boost. When pursuing you, make sure to dodge their short directional bursts. They can not change direction during them, so moving left to right relative to the Leech is the best way to avoid them, serpentine style.
 * If there are too many Leeches to use this strategy, then simply go fast by pressing Z underwater. Be careful though, because boosting underwater drains breath. This strategy is only viable in areas where long stretches of water don't have to be traveled underwater. In Shoreline, while traversing the long areas of open water where Sea Leeches stick in one place, you can wait for them to dive as far away as possible, press the jump button from a standstill on the surface to jump slightly out of the water, hold down, then press Z in rapid succession to quickly dive and swim under the water and bypass the Leeches. This is also useful in few other places, such as rooms in Drainage System that have pipes underwater in a pool of leeches.

Trivia
Jetfish will attack leeches in order to get food, but since they attack one leech at time, it is impractical to use this against leeches. []