DolphinsDolphins are marine mammals that sleep with only one hemisphere of their brain at a time, allowing them to remain partially conscious and aware of their surroundings while sleeping.
Sea OttersSea otters often wrap themselves in kelp or use other objects to anchor themselves in place while they sleep, floating on their backs in the water.
WhalesLike dolphins, some species of whales also engage in unihemispheric slow-wave sleep, allowing them to rest while still being able to surface for air.
SealsSeals can sleep both in the water and on land, often floating on the surface of the water while resting.
ManateesManatees, also known as sea cows, are large aquatic mammals that often sleep while floating near the water's surface or even resting on the bottom of shallow waters.
JellyfishWhile not in the traditional sense of sleep, jellyfish undergo periods of reduced activity and may float in a stationary position, resembling a resting state.
FishMany fish species exhibit periods of reduced activity that can be considered a form of rest. Some fish also find shelter in coral or other structures while resting.
CuttlefishCuttlefish are known to change color while resting, which could serve as a form of camouflage even during their sleep.
SharksWhile sharks don't sleep in the same way humans do, they exhibit periods of reduced activity and rest. Some shark species need to keep swimming to ensure water flow over their gills, allowing them to extract oxygen from the water.