A lovely picture, and congrats on 900th :> I'll give a little advice though on water when you're doing it traditionally. Even so I'm a digital artist, but colour rules and directions still apply.
Try to colour the water in a horizontal form, because the water is clear as the sky avoiding to leave any white marks (I know it's a tedious process, but worth the quality). Underwater, usually there if a form of a gradient, from dark to light, which would be from bottom (dark) to top (light) in deep waters of oceans and rivers.
For characters underwater, try to lightly, in the same horizontal, cover them in a very faint blue so it can barely be seen. There are lot of underwater pictures, mainly of sea animals, but they also appear different on land. So if unsure how, it's always good to use a photo for reference. =)
I... hope that kinda helps? Yeah I hope... xD But nonetheless I still like it~
Yea, having it in a consistent works out, but I personally wouldn't follow that exactly. Since giving the water a wave-like colouring gives it more life. Adds depth to it. Though gradient of the water helps a lot, lightest up top, darker at the bottom, in deep water. And if the water is clear in distance that should darken some too. A lot of things can play into it actually, positioning of the lighting, etc.