Forget sonotubes for anything near water. They are basically plain cardboard.
As a form, they are not easy to keep exactly round, but for your needs, they would work as forms (unless you want a lot of extra weight).
As a form, spiral wax paper around a tube and glass over that, then remove the tube. You can remove it easily once you get a hold of the cardboard and pull, because the tube is actually a coil.
Yes, they make sizes up to huge, so the sonotubes are still a good idea. You can even pinch the ends for bows, but you might leave a bit of cardboard within, but so what?
I suggest you work with no longer than 8 ft sections at a time, then join with glass tape.
Sonotubes are shipped nested, meaning 16", for example, may be slightly larger or smaller than you expect. Get the largest size, the outside tube, and get both the same size.