Your question is something that comes out of a Doctor Who Series!
To start with, could The Egyptian Sobek be placed in the category of a marine human hybrid.
Sobek (also called Sebek, Sochet, Sobk, and Sobki), in Greek, Suchos (Σοῦχος) and from Latin Suchus, was an ancient Egyptian deity with a complex and (literally) fluid nature.2 He is associated with the Nile crocodile and is either represented in its form or as a human with a crocodile head. Sobek was also associated with pharaonic power, fertility, and military prowess, but served additionally as a protective deity with apotropaic qualities, invoked particularly for protection against the dangers presented by the Nile river.
Then there is Triton:
TRITON was a fish-tailed sea god, the son and herald of Poseidon, king of the seas. He stilled the waves with the blow of a conch-shell trumpet. Triton was also described as the god of the giant, Libyan, salt-lake Tritonis. When the Argonauts were stranded in the desert he assisted them in finding passage from the lake back to the sea.
Trtion was depicted in Greek vase painting as fish-tailed merman, sometimes bearded, sometimes youthful. In Greek sculpture and mosaic he was often given twin fish or dolphin tails. Mosaic art added a pair of crab-claw "horns", green-tinged skin, and sometimes a pair of equine forelegs. As Poseidon's herald, he had a winged brow and conch-shell trumpet.
Triton was often multiplied into a host of sea-spirits called Tritones which were regarded as satyr-like daimones of the sea.
Then there is Skylla:
SKYLLA (or Scylla) was a monstrous sea goddess who haunted the rocks of certain narrow strait opposite the whirlpool daemon Kharybdis. Ships who sailed too close to her rocks would lose six men to her ravenous, darting heads.
Homer describes Skylla as a creature with twelve dangling feet, six long necks and grisly heads lined with a triple row of sharp teeth. Her voice was likened to the yelping of dogs. This description of Skylla is probably derived from the imagery of words associated with her name: namely, "hermit-crab" (Greek skyllaros), "dog" and "dog-shark" (skylax), and "to rend" (skyllô). In classical art she was depicted as a fish-tailed sea-goddess with a cluster of canine fore-parts surrounding her waist.
Late classical writers say that she was once a beautiful nymph who was loved by the sea-god Glaukos. She has a jealous rival in the witch Kirke who used her magics to transform Skylla into a monster. Older poets, however, imagined Skylla as a creature born monstrous.
Phorkys could also be added to this number.
PHORCYS (Phorkys) An ancient crab-like Sea-God. He was the father and grandfather of most of the monsters of myth.