Water on top of live sand will not kill anything in the sand. lol Not sure where people come up with this stuff.
if you already have livestock in the tank be careful when adding any sand, maybe make a paper funnel or something similar and try to shoot it down to the floor of the tank. Keep the pumps or powerheads off and it will settle in a day or so.
if you have no livestock yet, and you havent added the water then add the sand first...then add the water but pour it onto a plate you put into the tank so that youdont muck up the water with sand, the plate will help disperse the water evenly as you pour it in. Works very well, I always do that with my tanks.