What we have is a surprisingly precise set of reports and images that demand to be taken at face value. Doing that we have a sea adapted humanoid or even human being. Certainly the fish like construct in the image appears to be a likely adapter made small enough to not disturb balance. His return to the sea at night could easily have been to his habitat to even rid himself of his breathing harness. Reverse jointing on his limbs do indicate an alien humanoid.
There is nothing here we do not understand well. It is not even clear that these aliens are actually adapted to water. They may need the breathing apparatus to breathe our atmosphere as well as to operate under water. All that leads to a ship underwater and the amphibian routine.
Water Planets and Alien Amphibians http://nationalufocenter.com/?wysija-page=1&controller=email&action=view&email_id=111&wysijap=subscriptions&user_id=2771