https://jd.papermc.io/paper/1.20/org/bukkit/event/block/BlockFromToEvent.html#<init>(org.bukkit.block.Block,org.bukkit.block.BlockFace) There is a paper event for it that would give you the block that the water is flowing to. I think you'd need skript-reflect for it but I have 0 idea how but maybe that helps?
declaration: package: org.bukkit.event.block, class: BlockFromToEvent
