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. They will, however, respawn the next cycle. Leeches come in red and blue varieties.

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

Behavoir
When a living creature (excluding salamanders and other leeches) 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 drowning an entity they will stay latched on for a short time, then let the body float about and ignore it.

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, or a small centipede to electrocute them.

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 on a line perpendicular to the Leech is the best way to avoid them.
 * If there are too many Leeches to use this strategy, then speed up by repeatedly pressing the jump button 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 the jump button 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.
 * Allow a few Leeches to attach and have them fall off in a location where they will not pose a threat.
 * Squidcadas will actively "fish" for leeches by flying down to the water and allowing a few to attach, then flying away and dropping them near their nests. This, coupled with many squidcadas, can kill off the majority of leeches in a pool. However, the squidcadas will sometimes be drowned by all the leeches.
 * Swimming along the surface of the water provides some extra buoyancy which can help keep Slugcat above water when being attacked by leeches. Popped bubble fruit also provide a small amount of buoyancy.

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 a group of leeches.
 * Once the leeches are done feeding on their prey, they will return to searching for more food.
 * Unlike leeches in real life, they do not seem to suck blood or even consume their prey; rather, they leave it to float in the water and try to find something else to kill.