Apologies for my tardiness, I had unforseen bump at work and had to work over the weekend. Free time was spent away from keyboard. I have long weekend coming (Thursday is holiday here) ... Tai-Tai is coming...
Prone underwater would still make sense. Turned belly-up. Fins fouled or held, for those using legs for propulsion, spread or pulled straight so that you cannot easily move before re-adjusting...plenty of ways to put "off balance" effect without being literally prone (but you still use half your move to "get up"). It would just play differently (just as fortresses do, having walls makes no difference in 3d, it has to be dome or labiryntine warren of passages for protection...)