The forward buoyancy helps get through the waves, but if you're too far back, the boat will just flip over backwards. This is why you'll see alot of the crews straddling the hull in front of the crossbeam.

Your theory about powering up in calf deep water is sort of correct. When we're launching in the T500, when my ankles hit water, I'm jumping on the boat. We have pushers that continue to give us forward progress while we power up the sail. Remember the apparent wind generated from the pusher helps too - especially for steering. Another trick - if you're launching into the wind and you're having trouble getting stuck in irons... steer with your sails... keep your jib sheeted while keeping your main loose but still generating power. This will keep the boat from going into irons before you can get your boards down.

The issue that day was that the wind was blowing straight onshore and it wasn't blowing but maybe 10 knots. The surf was from a sub-tropical depression that was swirling offshore. The beach also drops off really quickly there in Jupiter - so the pushers had footing for 15 feet but then either got too deep to help anymore, lost their footing, or got smacked in the face by a wave. Basically we're doing the "powering up while standing still" but we're doing it while someone is pushing us too.