Water-bed mattresses are larger than regular mattresses, and because the mattress moves more fluidly, your bed sheets may slip off the edges and cause discomfort and irritation. With the advent of thicker mattresses, many different sheet styles are available including wider, longer sheets. Try using sheets made for thicker mattresses on your water bed.
Even if you're not handy with a sewing machine, you can add elastic bands to the edges of your regular or water-bed sheets with a needle and thread. They don't have to be perfect, just functional. Cut four lengths of thick elastic about 2 feet each, then sew the bands diagonally across each of the four corners of the fitted sheet, but leave several inches of space between the built-in elastic and your makeshift bands. Make the bed and loop the elastic around the edge of the mattress corners to keep the sheets in place. Premade bands are available from some retailers and attach to the sheets without sewing.
If your fitted sheets are still slipping off at the bed corners, purchase plain fabric and create larger corner pockets for the sheets. Hold the fabric up to the edge of the fitted sheet and cut a rough triangular pattern with one rounded corner for the rounded edge. Cut three more pieces to size and sew each into the edges of your fitted sheets. Make the bed as usual.
If your bottom sheets are staying on just fine, but you're tired of your top sheet coming out of the bottom of the bed, try sewing the two together along the bottom edge of each sheet. Place one end of the fitted sheet and against the bottom end of the flat sheet and sew them together along the seams. All of your handiwork will be hidden under the edge of the mattress when you make the bed.