one of the most difficult things about photographing underwater is your natural buoyancy. air in the bag and in your lungs can push both you and the camera up to the surface at the crucial moment. it can help to blow out a little air while underwater so your lungs are less full. this can allow you to stay suspended under the water. alternatively, try bracing yourself up against the pool steps or any other part of the pool that lets you hold your body under the surface.