The best way to learn is to look around, find something close to what you want for inspiration and try to emulate it updated with your own style. To get you started, make a sprite and add it to a layout, resize it to fit the bottom of the screen and add a nice water color to it. Add the water effect from the effects panel. Play around with the settings until you see something you like. Once you have that, make an animation or use particle effects to create bubbles, ripples, or whatever you want. If it's not to your liking, look around for another method. If you find one that you really like, try to contact the creator. Lots of times they will be willing to share their techniques.