11:30 is the earliest reservation available. I think that I read somewhere (or saw it here!) that there are 13 waterside tables available.
We showed up at 11:00 to let them know we were there, and then had to wait until 11:15, when they will start signing people in. We had to stand around until about 11:25, when they actually started calling out names, and letting people into the waiting area.
It was worth it - we were the second or third table seated, and got a great waterside table!
I think that we made our ressies about 45 days out, for the Friday before Labor Day. The parks were not crowded that day, but I do know that the Blue Bayou was not accepting any *day of* reservations for that day by the time that they opened!