I float the bag. I have never had a problem with the water mixing. If you are careful and can keep the water from the fish store out of the bag, you have nothing to worry about.
What I do is open the bag, put about a shot glass full of my water in the bag, then let the temp adjust. I keep adding water for about 1 1/2 hours then I take the bag out, catch the fish/snail or whatever I am adding, remove it from the water and put it in my tank.
I agree with tripper on the snails. I usually just do a temp acclimation them scoop them out of the bag and into my tank.