I have one of those flexible curve things. (Not sure what it's real name is since my hubby took it to his cabinet shop so he can use it to make a template for a counter top that will go against a curved wall and I can't look at the package to see the name.) It's about 18 in. long and flexes to any curvey shape you want. It is 1/4 in wide, so using it automatically adds your seam allowances. I would suggest getting one of those to make your quilt. You can change the curves through out your quilt making it look even more like water. I got it at a quilt show and it wasn't very expensive. There are instructions included. Your idea sounds beautiful. I hope you post pics when you are finished.