Because waterbed mattresses move (the "wave action"), hardside sheets often include features to prevent them from slipping off:
Standard department stores rarely carry these sizes. Look for these specific measurements when buying online: Where I Can Buy Waterbed Sheets Guide │Wholesale Beddings buy waterbed sheets
Purchasing waterbed sheets requires more precision than buying for a standard mattress because of unique sizing and specialized attachment systems. If you have a traditional waterbed (with a wooden frame), standard sheets will not fit. 1. Identify Your Waterbed Type buy waterbed sheets