Hey Grovylegirl. Here's just a quick paintover to show maybe what you might want to do.
Firstly, gather a lot of reference! Don't be shy about it! If you're stuck, there are tons of stock photos just waiting to help you out. I just googled "woman face submerged in water" and got some fairly good refs. For most paintings I do, I litter my canvas with all sorts of refs.
Also, for underlighting like you want... It's a bit hard because it really often doesn't look flattering on a face because it emphasizes the eyelids and nose with heavy shadows. Hair usually gets left in the dark, but I would opt for some rim lights just to make things pop.
Lighting/Value is seriously one of the hardest things to learn next to anatomy and perspective. It'll take a good bit of practice to get more comfortable with it, and I would always suggest trying to do black and white sketches/paintings to practice lighting because it really lets you focus on the value changes.
Hope this helps!