Did you use a high quality plywood for the floor? Given the choice I'd probably use a marine plywood for the floor but as long as you used real plywood and not wafer board you should be fine. The thicker the better. Is the floor raised off the ground by a few inches? If you are in a dry climate I'm not sure I'd put anything on the floor. Maybe deck stain or something like that. Sheds usually get a lot of dirt and debris on the floor so I don't think I'd put anything else down unless it is supposed to be a really neat and tidy garden shed. Our garden shed uses 1 x 6 flooring with a nail gap between each board so dirt and water can get out.