Puh, that situation is a tough one, but I think it mainly comes down to a taken out result. If you look at it, what it mainly does is take Molly and Marcone (and Thomas, I think) out of the fight with the concession that no more harm will come to them, and to give it a bit more substance, Harry makes a lore declaration, that he is prepared for just such an occasion. So it is a "no death result when taken out" sort of device, or at least used as such.
If you look at it like this, a simple maneuver spell wrapped in a potion (because you can declare those) should be enough. Activate on taken out, tag aspect, don't die. Maybe tie it to condition, like the crystal would shield you from death by hostile means, but if you where to activate it while drowning, it wouldn't do anything. And another item could allow you to breath and floats you to the surface, if you in fact are drowning. You would still be taken out, just not to the very far end.