The Cosmic Ocean is a twisted adaptation of every world in the game, with each level having the theme of a previous world. A player can find themselves back in a broken up recreation of Dwelling, Jungle, Volcana, Tide Pool, Temple of Anubis, Ice Caves, Neo Babylon, and the Sunken City.
Each level is always of varying width and height, and loops around within itself both vertically and horizontally. This effectively makes it so that if a player, item, or any other entity or object falls down in a level of the Cosmic Ocean, they will end up at the top of it, and going too far towards the left or right side of the stage will leave them in the opposite end.
Despite what the player may assume from their experiences with rope not functioning without a proper wall in the Duat, rope will function perfectly normally, even if clinging to the black nothingness of the Cosmic Ocean. Enemies that cling to walls like Grub will function normally as well.
Many of the level generation rules that normally apply in the standard areas do not apply in the Cosmic Ocean. Players can potentially be in danger the very second they enter a new level, such as a Giant Crush Trap appearing just above the entrance of a Temple of Anubis-type level or an Elevator appearing above the entrance of a Neo Babylon-type level.
The Cosmic Ocean is treated as an extension of Area 7, so the first level of the Cosmic Ocean is labeled as 7-5 in-game. This means that there are 94 levels that the player must beat to reach the 3rd ending of the game, culminating in 7-99, where they will become one with the cosmos and leave behind a Constellation.
Unlike the standard levels found in the rest of the game, a Celestial Jelly will be blocking the exit door, vaporizing any player that gets too close. In each level, 3 Celestial Orbs will spawn. When all of the orbs are destroyed, the Celestial Jelly will move away from the exit and begin moving towards the player in an attempt to stop them from leaving the level.
When the timer hits 3 minutes, a second Celestial Jelly will spawn instead of The Ghost.
Shops, Pets, Curse Pots, and Kali Altars will not spawn in the Cosmic Ocean under any circumstance. No Kali Altars or Pets makes the only source of healing the Kapala, and if the player is Poisoned or Cursed, the only way to remove the affliction is to die and resurrect with the Ankh (retained through a Skip).
When playing Co-op, player coffins will only spawn on levels 7-10, 7-50, and 7-90; death carries an especially heavy price in the Cosmic Ocean.
- If the player made it to the Cosmic Ocean with plenty of bombs and rope (possibly attained from the Corpse Bag awarded after completing the Sun Challenge), it is recommended that they make liberal use of them, as they will prove useful for making shortcuts through the level to the exit, climbing up to the bottom of the level, and other helpful uses.
- One should take advantage of the time that they have in each level, as 3 minutes is more than enough time to locate the exit, all three celestial orbs, and devise a plan of exiting the level after destroying the last Celestial Orb and triggering the Celestial Jelly.
- One strategy is to not destroy any orbs until the player has planned which orb should be the last one they destroy and have a solid route to the exit after doing so, while another strategy is to destroy all three orbs as quickly as possible, and deducing the location of the exit door by seeing which direction the Celestial Jelly comes from.
- When traveling on the edges of Cosmic Ocean levels, the player should become aware of any possible threats that may suddenly appear. This is because random falling enemies, objects, and projectiles can easily be a death sentence for the player.
- Although most falling enemies and projectiles cannot be stopped before they hit the player, carrying around the corpse of a Caveman or Cave Mole will prevent Spelunkers from being damaged by random falling Arrows fired from Arrow Traps, and the Pitcher's Mitt will prevent Rocks and Skulls from hitting the user, provided that their hands are empty.
- One should be wary of any potential hazards around the exit of a Cosmic Ocean level. Enemies, Crush Traps, and other threats may hide within a Celestial Jelly blocking the exit.
- Anything that can stun the player or otherwise deny them access to the exit, such as well-positioned Witch Doctors in a Jungle level, Croc Man or Cat Mummies in Temple of Anubis, or Forcefields on each side of the exit in Neo Babylon levels, should be accounted for before triggering the Celestial Jelly, so not to be met with an unfortunate death.
- The player should be extremely careful in levels containing enemies who can curse or poison them, particularly in Temple of Anubis-type levels, where both are present in the form of Cobras and Cat Mummies. Other sources of curse/poison include Witch Doctors in Jungle-type levels, Scorpions in Pots, Hermit Crabs/Pangxie in Tide Pool-type levels, and Arrow Traps in Sunken City-type levels.