Well, whenever I have done an open water swim... we ease into the water to a starting point, and then we swim when the gun goes off.

I don't walk during the swim... so I never worry about walking on the bottom then.

When getting out... you just have to suck it up. You will probably be OK though. I too have tender feet and somehow the excitement of the race takes over and I tune out the pain when I have to leave the water on a rocky surface.

Also... you could keep some flip flops on the side that you are exiting... and wear those to T1 to keep your feet covered on the way.

Good luck!