That cage is not too unlike real seal cages. They are used by researchers in Antarctica to study the animals. They are placed above seal holes, holes in the ice the seals use for diving for food and returning for air. I’ve even seen one documentary where they borrowed a seal for the day. They mounted a camera on him and took him to a place to help explore areas that human divers could not so easily do. It could not really escape as it had to return to the breathing hole that the researches had there tent set up at, it didn’t really seem to mind, they don’t seem to have too much fear of humans there. The scientists were even able to witness the seal doing something they had not seen before, it would swim to the bottom of the ice, where little fish were hiding in pockets, it would blow into these pockets and flush the little creatures out for a nice snack.