The Cat and the Mermouse (1949)

A day at the beach. Tom wants to lay in the sand, but his rest is disturbed by Jerry, who walks by to go fishing. Tom ends up falling off the end of the pier as he chases Jerry and lands underwater, where he encounters a mermaid whose top half looks just like Jerry. A chase, naturally, follows, all underwater. The mermouse, however, runs into a swordfish, and it begins chasing Tom, too, turning into mer-Jerry's ally, until it gets stuck in what looks like a telephone pole. The chase is next interrupted by an octopus, which grabs hold of Tom until Jerry yanks him free when we discover this is all a fever dream of Tom's, and Jerry has actually been performing artificial resuscitation.