You cannot place blocks when swimming.. this happens when swimming in a 1x1 block channel. You also cannot break blocks in a 1x1 channel.. it appears that the mechanics have you standing up even though it appears that you are prone because when you break a block in a 1x1 channel it always breaks the block above you not the block you intended to break.. it’s as if you were trapped inside the block above the 1x1 space and have to break it out before you can break a second block and even after that it still breaks the blocks above you.. This is for experimental.. I’m sure it’s the same for regular maps.