Ok, I'm not a lifeguard, or training to be one, but I am training for the Army, and I've looked at a lot of different things to get prepared, so I might be able to help. Stew Smith has a lot of stuff on Military.com, and this sounds like what you might want to look at.
To move through water, you have to pull water back with your arms, and have strong legs to propel yourself. Work on a rowing exercise, preferably Pull-Ups because you're moving your bodyweight, and do a lot of Flutterkicks. However, since you are not just moving yourself, you will potentially have to move an injured person, I would invest in some sort of ankle weight. Or use a small cinderblock on each leg, with the front of your foot in one of the holes, and the rest of the block supported by your shin, and do Flutterkicks that way.
http://www.youtube.com/watch?v=RR-ixFcc8Rw&feature=related (Try to have a good range of motion, and keep both of your feet off the ground about 6 inches when you're doing this.)
You should focus on Conditioning also, mostly because you will have to spring into action at a moment's notice. Try Interval Trainng, and do a few sets of Burpees on your workout days. For Interval Training, do a Run/Walk/Run thing, and cover some ground with your run. Do Sprints a few times a week.