While holding forward+sprint underwater(or holding forward with toggle sprint enabled), hitting an enemy while there is a block of air above you will cause you to briefly stand up then immediately reenter the animation for fast swimming. If you continue to hold forward after this, then whenever you go near a ceiling/anywhere there is a block right above you, you won't be able to move forward. You will stay stuck in place as your screen vibrates. You have to stop holding forward and wait until you stand up again before you can start fast swimming underneath blocks like normal. This is especially troublesome in ocean monuments.