In the depths of the seas and lakes of Rain World there is an entire ecosystem inhabited by extraordinary creatures that lead a completely different lifestyle from those that live on land. This does not mean that they are not dangerous, under the waters there are some of the most lethal and colossal creatures in the entire Rain World.
Jetfish are neutral, aquatic creatures found in and around 
Shoreline that propel themselves through the water at high speeds. They can be held and used as a fast mode of transportation through bodies of water.
Jetfish have black, medium-sized bodies and 2 bright purple eyes. They have shorter pectoral fins, longer tail fins with joints at the midpoints, and a vent between the tail fins which expels water. Their eyes are large with round pupils underwater, but constrict into a small, circular shape when out of water. Their heads may sometimes have whiskers.
When held, Jetfish propel Slugcat quickly through water and grant immunity to 
Leeches. 
Blue Fruit can be dropped on the water's surface to lure them up, granting an opportunity to grab on. They are easily controlled using directional inputs and allow Slugcat to safely pass through flooded tunnels and outpace aquatic predators like 
Leviathans. The force of their water jets can also propel Slugcat into the air.
Jetfish swim through water at high speeds, sometimes swimming near the surface and jumping out of the water using their water jets. They flop and jump around when on land.
Jetfish are completely immune to Leeches.
Jetfish catch and eat Blue Fruit, 
Bubble Fruit, 
Batflies, Leeches, 
Glow Weed, 
Lilypucks, and 
Squidcadas. These may all be used to lure Jetfish to a desired location.
Jetfish attempt to ram into Slugcat at neutral or lower reputation. This temporarily stuns Slugcat and knocks objects out of their hands. Groups of Jetfish can be dangerous, as they can stun-lock and drown Slugcat. Jetfish deal a small amount of damage every frame when ramming, making it possible for them to kill other creatures, including other Jetfish. Ramming damage occurs even if Slugcat is holding the Jetfish.
Jellyfish are white, saucer-shaped organisms with long, thin tentacles found floating at the water's surface. Despite resembling creatures, they are considered foreground objects, meaning that they reflect thrown Spears and Rocks (if the vulnerable jellyfish Remix setting is disabled), and can be pushed around.
When thrown, Jellyfish flash white for ~3 seconds, electrocuting most creatures that touch them during this period. Electrocution deals 0.1 damage and causes creatures to cease all function and shake violently on the ground. The intensity of the shock generally depends on the mass of the creature, with lighter ones experiencing stronger, longer shocks. Repeatedly throwing a Jellyfish to stun a predator and dealing damage while they are unable to fight back can make killing them much easier.
Stunned creatures cannot track Slugcat, making those few seconds a good opportunity to hide or get away. Stunned Vultures fly away upon regaining consciousness. It is also possible to utilize Jellyfish to evade Leeches, as they get caught in the Jellyfish's tendrils.
Giant Jellyfish are large, aquatic predators. They are found in large bodies of water, either swimming up and down underwater or staying immobile at the surface. They use their tentacles to grab onto prey and drag them in to deliver a lethal electric shock.
Giant Jellyfish are large aquatic creatures that have some similarities to their smaller namesake, the Jellyfish. They have a round body surrounded by a disc-shaped mantle, numerous tentacles of varying length and shape, and a large, circular electrical organ suspended underneath them that emits orange light. Their pale skin ranges in color from dull pink to an albino-like white. Their shorter tentacles appear as a chain of several rings, while their longer tentacles are thin tendrils. If underwater, their mantle expands and contracts slowly as a means of propulsion.
It is fairly easy to kill Giant Jellyfish floating at the water's surface using Spears, since only 3 
Spear hits are required to kill a Giant Jellyfish as 
Survivor. This can be done safely outside the Giant Jellyfish's reach using multiple Spears or up close with a single Spear. To avoid the Giant Jellyfish's tendrils up close, it can be helpful to jump out of the water and stand on top of their body, though it is easy to slip off as the surface is round and frictionless.
Giant Jellyfish detect prey through vibrations in the water around them, most often from entering or exiting the water. They are also capable of hearing sounds originating from out of the water and are easily distracted. If distracted by other sounds from above the surface of the water, Slugcat can swim below the reach of their tentacles. If their tentacles are dangling beneath the water's surface, it may be better to quietly swim over the surface of the water and try to climb over top of it.
Giant Jellyfish have difficulty capturing prey at the surface of the water. The grasp of their tentacles can be shaken off by jumping out of the water, and the top of their body is not electrified, so Slugcat may skim the surface of the water and climb over them. Alternatively, they can be avoided by diving underneath to avoid their tentacles. Avoid getting too close to the middle where the tendrils have an easier time ensnaring prey. Slugcat can also grab a Jetfish to easily move past them or use the Jetfish as bait to distract them.
The Giant Jellyfish delivers a strong electric shock to any creature that comes in contact with the bulb below their head. Giant Jellyfish at the water's surface are stationary, while Giant Jellyfish fully submerged in water swim up and down.
Once the Giant Jellyfish detects prey, they move their tendrils towards the creature in an attempt to grab them. If successful, they bring the creature closer to their center to shock and consume them.
A Giant Jellyfish may, rarely, deliver a nonlethal shock through their grasping tentacles.
Upon death, Giant Jellyfish burst into a pile of white blobs and 3-4 
Slime Mold from the orange glowing sack under their body.
Leviathans are massive aquatic predators found in large bodies of water in Shoreline, 
Subterranean, and 
Submerged Superstructure. They are the largest predators in the game, capable of hunting and eating any other creatures in the water.
The Leviathan appears to be biomechanical, having an organic body and a hydraulic mouth at the front. This mouth consists of metal rods that move the jaws and large grooves that allow them to clamp down onto prey.
Their body consists of progressively thinner segments that end in a long tail, with large fins and smaller frills of varying lengths along their sides. The former are generally longest at the front of the body, while the latter tend to be longest toward the middle of the body. A gradient runs across most of the body in a marbled pattern, ending near the tail. Their head displays patches of color from the same palette.
Leviathans cannot be practically killed in normal play due to their large health pool. Avoiding detection is also difficult due to their large size.
Leviathans can outpace a swimming Slugcat, making outmaneuvering them near-impossible in open waters without aid or knowledge of swimming techniques. If possible, it is best to avoid large pools altogether, or wait until they have eaten enough food to become dormant for the rest of the cycle. If passing by one is necessary, the safest way to do so is by getting hold of a 
Jetfish, as they are fast enough to swim past Leviathans safely.
Leviathans have extremely good, omnidirectional sight, somewhat limited by the surface of water but not by water itself, making them able to near-perfectly see the creatures around them. After noticing potential prey, a Leviathan moves towards it, extending their jaws out when close enough, and clamping down on it, consuming whatever is caught in their jaws. Leviathans only hunt in a room for a certain amount of time, effectively turning passive and leaving after that amount is reached.
