(image source) Most mammals' trachea (windpipe) is connected to both their nose and their mouth, so the animal can breath through either their nose or mouth. read more
Baleen whales have two blowholes, but dolphins, like other toothed whales, have only a single blowhole. So if you blocked off a dolphin's one blowhole somehow, it would suffocate. But I don't think a cork would do it. read more
And the dolphin would eventually batter the croc to death, as they do sharks. A crocodile too large to kill this way they would simply flee from. Of course, one bite from a big crocodile would destroy a dolphin, so in an ambush, bet on the reptile. But in general, I’d put money on the dolphin. read more