S01E72: Moomin and the Dolphin

Baby dolphin is washed up on the beach one day, and Moomin and Snufkin have a tremendous job getting it down to the water again, because even a baby dolphin is far bigger than a Moomintroll. A year later it returns, grown-up now, and lets Moomin and his friends ride on its back. It also invites Moomin for a trip down to the bottom of the sea, and is very surprised to hear Moomins can't breath under water. But Moomin is consumed by a desire to see this other world, and asks the witch if she can't think of a way...