It's a touchy subject, but I don't believe any species can't be kept in captivity, it's just a case of a facility willing to construct the right enclosures which would be a mammoth task, but not impossible IMO.
And Cat-man, couldn't disagree more about the Whipsnade sealion pool as it's one of the largest in the UK: Yes it's old fashioned, but it's got a massive water volume, a big outdoor pool with a bigger indoor one, and they are given a lot of enrichment and they do the shows, so they're always busy.