As far as speculation goes, I think penguins are a dead end. They're too specialized, as far as I can tell, to survive the environmental changes that would otherwise make them into a full-time ocean animal. Here I'm assuming the land breeding locations they need to breed and rear chicks to be unavailable.
Amphibians or other non cetacean mammals (like seals) might be able to make it back to the water full time.
Edited by CosmicChimp, : changed heading