Giant Sharks.
Kraken (Who are surprisingly polite, though a bit grumpy)
Four kinds of shape-shifting animal-people who shed their skin to become human (Selkies to the north, Serpent-People to the south, Swan-Maidens near lakes and rivers more than the sea, and Fish-People all around.)
A variant of centuar with a lower half similar to a Plesiosaur. They tend to be rather insular.
Deep-Dwellers, called Sea Trolls at times, they live in the deep darkness of the ocean. Very strong, but also unable to travel to far up.
Sea-Serpents, but they're more like rather smart dogs that anything threatening.
And there are some Cephalopod-ish people. Looks like a merperson with an octopus end. Tends to have translucent skin. Very, very smart.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather