In most video game levels, a playable character can run and jump freely. Not so much the case when when a level forces them to dive into water (be it a lake, a sea, an ocean, or some other body of water) or another liquid.

One of the most common features of underwater levels is the introduction of additional obstacles and complications to player movement, with three being especially common:

• Swimming requires a different control scheme from that used for the standard run-and-jump mobility
• Moving through water or any other liquid results in slower movement, at least without a power-up or other aid.
• Unless the game invokes No Such Thing as Drowning, then there's the risk of drowning if the Oxygen Meter is depleted.

Aquatic video game levels have been a staple of Platform Games since Super Mario Bros., but they can be seen in games from other genres as well. Typical underwater enemies are sharks, octopuses, squid, electric eels, jellyfish (and sometimes Electric Jellyfish), clams, and some common fish like basses, rays, and mackerels with basic swimming patterns to serve as underwater Goombas if the former options are a little too threatening. These areas typically have a generally warm-tropical ocean theme, with lots of coral and seaweed as obstacles or decorations, with some Underwater Ruins every now and again as well, and with enemies based on sea life. Other environments, such as arctic oceans (usually some form of mixture with ice levels), lakes (which may have crocodiles or piranhas as enemies instead) or flooded dungeons also appear in games, but tend to be less common.

Environmental hazards primarily take the form of a limited Oxygen Meter, sometimes paired with areas where it's difficult to surface or replenishable by using Oxygenated Underwater Bubbles, alongside themed variants of the classic Spikes of Doom in the form of beds of razor-sharp coral, spiky shells, or clusters of spiky Sea Hurtchins. Less frequently, the character might have to deal with underwater currents that make mobility more difficult. Frequently, particularly in adventure games, metroidvanias, and similar genres, a special item such as a set of flippers, a diving suit, or a particular spell will also need to be acquired before underwater areas can be accessed.

Sometimes, a little artistic license happens. Human heroes, such as Mario, often swim fully clothed, without being weighed down or otherwise impeded—it's as if Water Is Air. The character might suddenly acquire a snorkel upon going underwater, ready to partake in Submersible Snorkeling, as though a snorkel wasn't just a plastic tube that needed to be partially above the surface. While many games will penalize you for running out of oxygen, they generally won't simulate the bends or punish you for surfacing too quickly.

These can enter That One Level territory, depending on how difficult it is to swim or how small the Oxygen Meter is. Closely related to Palmtree Panic, Gangplank Galleon, and Tide Level, given the prominence of water in those locations. Levels that fall under Down the Drain are especially prone to featuring special water-based hazards and hindrances, as detailed on that page. For works (including video games) that take place underwater and use the setting as part of the worldbuilding, see Under the Sea. If the level takes place within a facility built underwater but you don't swim while exploring it (for example, a submerged military base or a laboratory), it's an Underwater Base. If a boss battle takes place entirely underwater, then it's an Underwater Boss Battle. See also Underground Level and Lava Is Boiling Kool-Aid.

Examples: