How about following swimmer to the pool, then down a ramp into pool with rig (lo-mode), maybe ankle-deep into water. You'd be leading the swimmer a bit. As swimmer dives, you arm down the camera into water FORCEFULLY, so there's a good splash of water around camera. Use that splash as a "hidden cut" opportunity to mimic same move in a 2nd shot, this time w/o rig, only camera in housing, so that you can continue shot underwater. I'd think the moment of impact would be visually jarring enough so that you'd miss the cut (well, soft cut, maybe 2 frame dissolve). Easy enough to test by simply doing two takes with camera in housing & see if they cut.