You can post all kinds of animal species which are part of the marine life, whether ocean fishes, marine mammals, sea birds, sea turtles and reptiles, cephalopods (octopus, squid, etc.) and other sea mollusks (shellfish, sea snail, etc.), marine echinoderms (starfish, urchin, sea cucumber, etc.), crustaceans (lobster, shrimp, etc.) and other marine arthropods, cnidaria (jellyfish, coral, etc.), etc. Zoological names are helpful to find the correct pictures and information to share (and for "problematic letters", especially X, Q, U and Z).
Who doesn't love Nemo, an anemonefish, or, more precisely, a clownfish (Amphiprion)?
This is for A ...
Who takes the B? 
Anemonefish (picture: ocellaris clownfish, the species featured in the film Finding Nemo)
@Dan
And here's one of these black "sea devils" (there are different kinds of anglerfishes):
It is not as suited to be a main character in a movie
Lol - indeed!
Brown Trumpetfish
Blue Devil
Crayfish
Thank you, Dan; the picture is great!
Hello, Dan, how about adding a marine creature?
E is on!
Dogfish (Spiny Dogfish)
https://en.wikipedia.org/wiki/Spiny_dogfish
Eel (Yellow Moray)
Hello, Midnight Cowboy, how about continuing in this game?
Flounder (European Flounder)
(Platichthys flesus)
Green Sea Turtle
Hammerhead (Great Hammerhead Shark)
(Sphyrna mokarran)
Thank you, Dan!
Hello, Dan - are you chuckling now? Here's another challenge: a marine animal beginning with "F".
Jawfish (Opistognathus aurifrons, yellowheaded jawfish)