I absolutely despise the ocean, but i love the sealife there!
I would personally like to see in no particular order (some are redundant to previous posts):
-Anglerfish (Maybe see their lights at night rather than the day considering its a deep sea fish.)
-Frilled sharks, or sharks in general
-Spider crabs
-Coelacanth (Rare spawn, but a nice addition since they're well known!)
-Shrimps (Maybe a fishing basket to catch and farm them? Tasty sea roaches~)
-Dolphins (Hate these fuckin things for a multitude of reasons...)
-Pufferfish
-Viperfish (My personal favorite fish)
-Blob fish (Funny fellas, look up how they look when they aren't crushed by the depths when they die and float to the top.)
-Smelts/Sardines
-Tuna/Mackerel/Salmon/Cod, etc
-Swordfish/Sailfish!!
-Perhaps types of clams? Clams are nice! They could also have a low chance to give pearls to craft pearl necklaces.
-Moray Eels!
-Oarfish! Lots of sea serpent myths come from these guys
-Octopi (Blue ring, Dumbo, Mimic, and Umbrella specifically.)
-Giant squids (Kracken bois)
And finally Seahorses! I love seahorses so much. And an idea i've always liked was to make them a sort of sea mount, although that might be difficult since mount code isn't readily avaliable for Vintagestory quite yet as of writing this.