Mermen rule the seas
If I was to live underwater, since this is clearly hypothetical and open to fantasy, I would be a merman.  Most mermen throughout mythology were fearsome c creatures to be reckoned with.  They are always handsome, from the torso up that is.  They are uniquely strong and swift.  As a merman I would freely rule the sea.  I would be the smartest creature in the water universe and therefore be able to adapt myself to stay on top and out of the jaws of anything bigger than me.  Furthermore, I would not have to change my idea of beauty of thinking that much, as opposed if I was to take on the form of some lesser sea faring creature.