In case if you don’t want to draw mermaids but have troubles with coming up with ideas what else to draw.
Marine monster - for ideas you can go to @seatrench - they have tons of awesome. Or draw any marine animal/bird to your liking that you’ve found there.
Rusalka - fresh water spirit of those who drowned. female ones tend to try to drown other people and marry Vodyanoy (deity who watches over all Fresh Water Domain).
Selkie - a wereseal who can take off their pelt. Also don’t be an asshole - don’t steal their pelts;
Swan-maiden - similar to above, only swan instead of seal.
Waterhorse - fav means of transportation of the guy mentioned above. They usually pretend to be an exceptionally big catfishes. Unlike kelpies however, they aren’t assholes and typically they shy away from humans. Also, “horse“ part is up to discussion.
Reverse mermaid - upper half is a fish, human part is a bottom one;
Fishman - mix of human and fish features;
Kraken - everybody’s fav giant squid;
Hippocampus - half-horse half-fish ( you can go for half horse half-dolphin if you want them to be fully mammalian);
Siren - sea-dwelling half-birds half man-eating monstrocities beautiful maidens with enchanting voices;
Ichtyocentaur - basically think of centaur mermaid: human torso attached to a horse body which has fish tail;
Capricorn - half-goat half-fish, sadly very rarely seen outside of zodiac drawings;
Kelpie - nasty water spirit who pretends to be a pretty horse, often white or black, to lure their prey to ride them. After person tries to ride them, kelpie becomes sticky and person can’t get off. They are said to eat everything but liver;
Tarbh uisge aka water bull - creature similar to kelpie (water dwelling shapeshifting entity) only a bovine and a lot friendlier;
Dobhar-chú aka water hound - It resembles both a dog and an otter though sometimes is described as a half dog, half fish. Is said to have fur either way. Lives in fresh water;
Ahuizotl - dog-like creature with spiky waterproof fur. It has hands capable of manipulation and an additional hand t the end of its tail. The ahuizotl is feared due to its liking for human flesh, especially nails, eyes, and teeth. It is said to live in or near the water and to use the hand on the end of its tail to snatch its prey, dragging the person into the depths to drown them;
Lavellan - poisonous water-dwelling rodent who can somehow injure cattle hundred feet away;
Hafgufa (from Icelandic: haf “sea” + gufa “steam”) - giant sea monster, who can disguise itself as an island or pair of rocks rising from the sea;
Lyngbakr (from Icelandic: lungi “heather” + bak “back”) - colossal whale-like creature who can swallow ship whole.
Morse aka sea-lion - half-lion with webbed paws and a fish tail;
Charybdis - because nothing is as vague as thing one: some sort of giant creature, who lived under a rock at one side of a narrow channel. Three times a day she/it would swallow water, creating a whirlpool. Nobody knows how exactly she looked like.
Scylla - oh wait, this one is! She lived an arrow flight away from Charybdis. She could snatch 6 people at once. But in short - noone exactly know how she looked like, however here we at least have a version.
Scylla was said to be a gorgeous young woman. So one day Scylla was bathing in the sea and a sorceress named Circe happened by. She saw Scylla and got extremely jealous of her beauty. So Circe poisoned the sea water which caused Scylla to transform into a frightful monster with four eyes and six long snaky necks equipped with grisly heads, each of which contained three rows of sharp shark’s teeth. Her body consisted of 12 tentacle-like legs and a cat’s tail, while six dog’s heads ringed her waist.
Another version says that Scylla had upper part of a woman and bottom of a dog.
Well, her looks are up to you. *dramatic shrug*;
Sea monk or monk-fish - fish that looks like a monk. Do anything with that;
Bake-kujira - a large ghostly skeleton whale that is said to be accompanied by strange birds and fish.
Makara - as the vahana (vehicle) of the river goddess Ganga, Narmada and of the sea god Varuna. Makara are considered guardians of gateways and thresholds, protecting throne rooms as well as entryways to temples.
Typically it’s half terrestrial animal and half water animal: for example - head of elephant and body of fish or crocodile. Or body of a fish, trunk of an elephant, feet of a lion, eyes of a monkey, ears of a pig, and the tail of a peacock.