It would be interesting to see an exhibit modeling a sandy beach, which wild seals and sea lions in California frequently use. I wonder how practical sand as a waterside surface would be, though - thinking about both sanitation as well as water quality and visibility.
Sand is tricky in such exhibits because it can easily get up into the water column and get into the filter and cause issues, as well as impact visibility. It's also virtually impossible to clean effectively in that sort of situation.
The only wish would be a soft surface on the land to rest, which pinnipeds prefer - but the zoo could easily provide it.
Harbor and Grey Seals certainly do - not so sure about California Sea Lions. I've seen wild sea lions all along the coast here and I've never seen one on a sand beach, even when they were readily accessible. Always been on rocks or occasionally docks. Harbor Seals I've seen lined up touching side by side to make the most of a small sandy beach.