It was part of a collection, a series of short animated movies

A boy and a girl are on a boat in the middle of the ocean, a fish comes up to their boat and offers them a device (it may have been seaweed or netting) that would allow them to breathe underwater. They travel down and watch the lives of the fish, I think a royal family may have been viewed.

That's all I remember, any help?

Land of the Lost (1948) (http://www.imdb.com/title/tt0151293/)
Land of the Lost Jewels (1950) (http://www.imdb.com/title/tt0151294/)
Land of Lost Watches (1951) (http://www.imdb.com/title/tt0151292/)

Again, thank you!

Glad to help :)