Over at Animation Backgrounds there is a post about the backgrounds used in Merbabies. While this is a Disney short, the boatload of the work was done at the Harman-Ising studio, due to Snow White taking up everyone's time. Harman and Ising worked for Disney back in the Oswald/Alice comedy days.
Animation Backgrounds also has several posts from the Little Mermaid (not enough undersea shots IMO), a few of the underwater panels from Bedknobs and Broomsticks (I'd love to see more) and while I didn't find any from the undersea (or 'in the whale') portion of Pinocchio I'm sure it'll happen eventually! Honestly if you have even a passing interest in animation (as in, you watch it) you should check out this blog! What you'll see will amaze you!
See Merbabies and Waterbabies on my blog here.