Octopodes, crabs, shrimp, and a few fish just straight up walk on the sea floor. Skates and rays, as well as a few other types of fish, don't "walk" per se, but do scoot along the bottom by pushing themselves with their fins. And that's just real life!
One of the illustrations this chapter is a mermaid that's a regular human with a clam for a head. He won't have much choice but to walk, unless you expect him to swim by jetting around with his body flapping behind him like a piece of toilet paper stuck to a shoe.