I'm thinking of doing something similar with pinkie, but I have yet to put together a cohesive storyline together, haha!

But anyways to help with your practice, with the bubbles and her tail, it seems like she's floating down. But her mane seems to float downward, which seems to suggest that she isn't in the water at all. I'd suggest you watch some underwater videos of people sinking through the water and swimming upward to get what I mean with the hair. Good luck!