Not just Mobs.. Oceans are big, big places. cities, instances, missions, treasure hunting.. so much and so many things to do in the water. And I would kill for a mini sub to go exploring in.. We can fly, we can drive, and we can boat,, Why not Sub
To have big sea mobs Ark shud have more sea too and that mean at least couple of servers more and that depend on MA decision so it will not happen so soon.