Waterbed sheets are only attached to each other in the middle of the foot of the bed. They do not have the corners sewn together. In the corners they have elastic bands that plastic bars slide into to hold the sheets in place. The plastic bars run against the sides of the frame so it holds the sheets in place.
I also use deep pocket king sheets (king waterbed) and they never come undone.
I can go up and measure the seam on my waterbed sheets and can take pictures if you would like.
PM me for more info and pictures.

That is how I made ours. I sure miss that bed. It was always warm...especially in those cold Iowa winter nights.

I use regular CA King sheets on my waterbed. The bed is a CA King and I have no problems. I don't keep the mattress tightly filled with water cause we don't like it completely filled. I just make sure that the bottom fitted sheet is rolled under the mattress fairly good. I don't like a top sheep sewn to the bottom so I just tuck the bottom and about a foot up on the side. They never come loose. I do like the deeper pockets in the fitted one. Besides if the sheets are sewn together they are harder to get in the washer and harder to fold.

You are bring back memories. When I would make mine, I would use King sized sheets. Sew the top sheet to the bottom sheet, but NOT all the way across the bottom. I used to leave between 5 to 8 inches open on each end (just like the store bought sheets). I also used to add elastic straps on the under side of each corner (this helps hold them on as the water moves around). Also remember to place the top sheet facing down when you sew the two sheets together. Good luck

I used regular sheets on my waterbed. Just used the elastic suspenders to keep them tucked.
http://www.ebay.com/bhp/bed-sheet-straps

I would think two sheets sewn together would be very difficult to handle.

I too used regular sheets. In fact I bought one set that was stitched together, and unstitched them. I didn't want that much underneath