I guess it depends on what kind of floaters you have. For lettuce and hyacinths I know several people who use a hullahoop or a styrofoam circle. The hullahoop is quickly covered but I think they paint the styrofoam to hide the white.
you could use fishing line and lightly tie it around the plant and then put a weight on the end of the line and let the weight sit on the bottom. The plants can still move like they normally would, they would just be on a leash.