I cannot remember the title. All I can remember is it was a fantasy book involving a girl who was given the power to swim underwater where she befriended a whale, a dolphin, and it involved rescuing her friends at the bottom of the ocean who were trapped there underneath a bubble. One key character that I remember was a “dimity bird” that was enchanted to move about. Presumably published in the ’60s, early ’70s.
Any chance it is Deep Wizardry by Duane Duane? https://www.goodreads.com/book/show/176494.Deep_Wizardry