I have spawn proofed a a water monument by creating columns of solid blocks to the bottom of the ocean other than in one spawn location then created a bubble elevator using soul sand to raise guardians up to a kill chamber to farm for prismarine. The guardians however will not spawn over the soul sand block and will only spawn in non flowing source water. According to previous versions of the game this is a bug but maybe its a parity change hard to tell as this maybe is just how java have always had it and now we have it too