I recently bought a water bed and all of the waterbed sheets that I have seen for sale have to cover sheet sewn to the fitted sheet. As a person who doesn’t like my sheets tucked in I have to wonder, why in the worl would they do this?
Possibly because your tastes are unique? I rarely use a top sheet myself but when I do I never tuck it in the sides either. I do tuck it in the bottom because it keeps the sheet from bunching up and makes it easier to make the bed without producing the confined feel that I dislike about sheets that are tucked in.
I do have a suggestion though. Buy a set of waterbed sheet holders and use flat sheets instead. They work like large versions of women’s garter belts and are far superior to waterbed sheets anyway.