Temperature and depth are your barriers in the ocean. Water can be shallow and swimmable but if it's too cold to live in for a species it may well be a brick wall.
But broadly, the distribution of thermal habitat isn't very complex, it varies across a latitudinal cline and at a broad regional scale with ocean currents but your don't have small island-sized patches of warm water for example. Most fine scale anomalies are short lived especially relative to the scales speciation happens over.
Depth is a thing and can separate disparate communuties on seamounts like alpine species on mountain tops. The difference is most Maine species have a pelagic larval stage and can disperse widely, so barriers to gene flow are more limited than on land.
Depth is a thing and can separate disparate communuties on seamounts like alpine species on mountain tops. The difference is most Maine species have a pelagic larval stage and can disperse widely, so barriers to gene flow are more limited than on land.