First, u take 1 skulk sensor and place a. iron trapdoor in front of it, Skip 1 block and place another iron trapdoor and behind the trapdoor, place another skulk sensor now waterlogs all blocks and cover it completely with blocks. to get an output u use a comparator and then a repeater behind the block that covers the skulk sensor. And there u have it. I think it needs to be patched but it is a nice feature after all.