Isn't Cthulhu a god/alien? He may currently in the sea, but he's not a "creature of the sea" - and will not always be that way.

Cthulhu is a Great Old One and is by far the most prominent member of the group. He currently lies in death-like sleep in the sunken city of R'lyeh somewhere in the Southeast Pacific Ocean. "When the stars are right", R'lyeh will rise from the sea, never to sink again, and Cthulhu will awaken and revel across the world, "ravening for delight" (src).

He's also not necessarily fully "aquatic" biologically: "simultaneous pictures of an octopus, a dragon, and a human caricature". And I'm not sure that Octopoid physiology fully implies aquatic evolution.

So the answer is "NO".

Please note that "Great Cthulhu is one of the Water Beings" is from Derleth's scheme, NOT from Lovecraft himself.