There are several different types of aquatic beings on the planet.
There are five different types of aquatics in the Super Federation Council. Three of them have tails instead of legs. Some of these tailed beings look mamalian and have walrus-like tails while others are more eel-like. “... they are maintaining a large number of experiments in our oceans, and consequently do not like humans very much. They consider us a cancer to this planet's oceans and wildlife.” The green seamonkey-like aquatics reside by the Korean peninsula.
They have bases on the bottom of oceans and super lakes. They travel to and from planets primarily using portals.
The being featured in the documentary Mermaids: The New Evidence exactly resembles one aquatic species.
“There have been reports of Asian cultures and some of the ancient breakaway civilizations being in contact with some of the aquatic beings going back thousands of years. ... There is quite a bit inside the giant and sprawling underwater lava tunnels close to Iceland (Just to name one, most places [with] underwater volcanic and other useful geology have some sort of currently or formerly occupied structures).”
There are some aquatics who feed themselves by rubbing algae into the gills on their chests. “... that suffices as a meal for them.”