If you have two water sources next to an air block, they will create an infinite water source between them. However this doesn’t work if they are next to a waterloggable block. This allows you to get air underwater, like before the aquatic update.
- Build a cross underwater made with non-waterloggable blocks
- Replace the block in the middle with a slab
- Break the 4 adjacent blocks on each side of the slab
The slab won’t get waterlogged and you have air underwater.