Waterlogging a block that can be moved by sticky pistons, then retracting the sticky piston vertically with the block attached, the water will disappear server-side but not client-side.
- Place a slab
- Place a sticky piston above and facing the slab with one block of separation between them
- Waterlog the slab
- Activate the sticky piston
- Deactivate the sticky piston
Here, a single water source will appear under the slab. Reloading the chunk/ world makes it disappear. Items do not float and entities cannot drown in this space.