[J. V.]

Sand and St. Augustine are a match made in heaven. It drains and St. Augustine sod is grown in very sandy soil commercially. I would do my best to get the rocks out, level the sand with a roller, broadcast some safe organic fertilizer like Milorganite and lay the sod.
Make sure you tuck in any raised edges. Do not let the edges sit above grade. It looks like you need not worry about this from your photo. Its a perfect spot for sod.
Water daily until it gets started. Do you have irrigation (sprinklers)?

I was born and raised in S. Florida and St Augustine is the standard. Its very easy to grow and maintain. In fact I have seen sand brought in just for St Augustine grass. Sand is a positive and not a negative. Your sod will do very well in well drained sand. Its the very best medium for St Augustine. Thats why the sod producers grow theirs in sandy soil.

St Augustine is a very hardy grass. The only issue we had back home was chinch bugs. But its easily remedied. St Augustine is beautiful. I used weed and feed on mine and I had the best looking yard in town!
Here in SC Centipede is the standard. Its very similar to St Augustine. The blades are thinner, but other than that they are close relatives. I have 2 acres of beautiful centipede I started working on years ago.
I put in plugs often every fall and sometimes in the spring. It took all these years but my yard is 90% centipede and I have yet to use any weed and feed on it.
Good luck. You are set up for sucess. Don't worry one bit about the sand. It is the best medium for St Augustine.