I have already drawn a prawn, a goldfish, a seahorse and a crab as well as one sea shell (that counts as one). I really need another creature to draw urgently!! I can't think of any that could be good to draw in a space half the size of an A4 sheet of paper.
Starfish, squid, sea urchin, water snails, jellyfish, are just a few examples.
My suggestion would be to use google images and choose one that you like the look of.
You are able to draw anything on an A5 sheet of paper, if you try. So don't hold back on an animal you wish to draw. :)
i advise you to link it to an artist that you've already fond which links to your theme of marine life......... a perfect example could be that you use 'Ernst Haeckel'. I had to research him for Art GCSE and he does astounding detailed drawing and records of all the marine life he comes across in a distinct style. I'm sure he would be a very good artist to start off with! An as for which sea creatures to focus on; i'd advise you to just have a look at his work and see which one you'd prefer using in your coursework. Hope this helped, all the best x jenny