We own a waterbed and they are notorious for being difficult to keep sheets on. It is also hard to find sheets and we try to use the same two sets for as long as possible.
Before I discovered sheet straps we had to use ingenuity.
I sewed two loops of elastic on each corner of the mattress pad. My husband cut PVC pipe into 8” pieces and we put them in the loops. The weight of the mattress kept the PVC pipe secure and the loops kept the sheet or mattress pad attached to the PVC pipe.
NECESSITY IS THE MOTHER OF INVENTION!