Realism? Well, actually all of the marine animals named above are kept in captivity. Not a question of realism, but of the conservation and animal-welfare message. I don't mind and actually want seals and walrusses, though.In my opinion, at least these animals can be kept somewhat "okay-ish" in a zoo.
Indeed, most of the pinnipeds are fine in captivity as they are far easier to keep stimulated than cetaceans and don't require as large of an area to be happy in. I'd put pinnipeds on par with elephants in terms of animal welfare in zoos; it's never going to be perfect but it can be good enough.
The main thing I take issue with is the zoos that
only have them for shows. I can't remember where I was at the time but there was one zoo I went to that had wonderful habitats for their African species herd animals, big carnivores, and chimpanzees, but their sea lion display pool was far too shallow and basic and I wasn't even sure it was saltwater. The primary attraction for the sea lions was their show, which occurred twice a day. Outside of their shows, the sea lions were not kept in ideal conditions.
Conversely, Auckland Zoo here in NZ has rescued New Zealand fur seals on display in their award-winning
Te Wao Nui exhibition showcasing New Zealand fauna. Their pool is big, interactive, and has variable terrain, and their 'tricks' are designed to teach people about conservation (such as picking up litter from their own pool and putting into bins). They used to have California sea lions but once they died off they started taking in rescued fur seals instead.