Artificial gills

What if you could stay underwater indefinitely?  That may be a future possibility with the claims of this invention.

One thing that you may consider is to use it for space travel.  How?  Swim inside a water filled cabin that provided shielding from cosmic radiation.  There's a lot more shielding from water than from air.  The air would have to be replenished from some source, but that's done anyway.  If mass can be saved this way, why not?

Somebody on the internet, which I cannot cite, called a human's skin as a spacesuit for a fish.  What if you can borrow an idea from a fish in order to settle the solar system?  Water can be a spacesuit for a human, in which one is equipped for life in the water as in the way of a fish.

Movement inside the cabin requires one to push water out of the way in order to get somewhere.  This may help with muscle tone.

Downside?  The mass of course.  Fill up a Dragon with water would require 10 cubic meters.  That's 100k kg, or 220k lbs.  Mass is the big problem in space.  To move around that much mass would require some serious thrust.