large-brained whales have been around for a lot less than 30m years
Earlier big whales weren't that bright. And cetaceans aren't stagnating when it comes to speciation (as opposed to the one-terminal-branch hominids) so that creates more opportunities to get ever smarter. They've got 500m-1G more years to come up with something before oceans boil, so I wouldn't rule them out here or on other planets.
And again, there will be surfaces to stand on even on all-water planets.