Well you could do a 4 parter:
#1 is starting from the sea level and going down (maybe the top being a pier and you jump off it). On the way you see fish and sea creatures (could use this - How to create Semi Realistic Fish guide)
#2 is a bit darker version of that, having different fish and animals
#3 gets dark, with sunken ships and treasure lying around, with little life at this point
#4 is pretty much entirely black, with some lights of the anglerfish scattered around.
Hope this helps! Sure! Just put my name for the credits 
Yes ‘Aguero10’