C.C.
I was a lifeguard all during my teen years and was on swim team forever. I can tell you that contrary to popular belief, people who are about to drown do not thrash around and yell for help. What you look for is a really big set of eyes on a face that's nearly underwater - that's the person who's about to go under and needs to be fished out ASAP! Keep your eyes on the pool at all times; even kids who are good swimmers can get a leg cramp that will put them underwater in a matter of seconds.
How good of a swimmer is your stepdaughter? If she is on swim team or the like, then she will probably be ok in the pool without an adult right next to her. However if she is not able to confidently swim across the pool alone, at the very least she needs to stay well within the area where she can stand up.
If she does stray into the deep end and you need to rescue her, if you do not know how to swim, don't jump in. Most pools have a "shepherd's crook" (a big hook on a pole) that you can reach out for her to grab onto. Likewise you can use anything that floats - a kick board, rescue tube, anything like that. You could even sit on the edge and reach out your foot to her, or you could hang on to one end of a beach towel and toss the other end out to her.
If the other kids get too rowdy, have your little one get out, and ask the parents of the other kids to settle their kids down. You don't want one of them jumping on her in the water! That kind of situation can get out of control fast.
Other than that, no running, no diving, and have fun! =)