Poolrunning works so much better when you have the toys to support it. Here's what I use:
I use the Garmin 310xt. It's my normal GPS / HR monitor training watch that I use when running outside, but with a few extras that also make it very well suited to poolrunning. For one thing, it's water proof, which is obviously key.
For workouts, you can program it to provide you "alerts", by either vibration or tone, at set points (by distance or by time). I use this function a lot in the pool. For my "interval workout", I program the Garmin for "10x3:00 interval/1:00 rest" -- essentially 10 intervals of 3 minutes each, with 1 minute rest. The Garmin vibrates on my wrist at the end of each 3 minute interval, and vibrates again at the end of the 1 minute rest to cue me to the next interval. Thus, I can skip staring at the pool timer as I do my workout (and also can still do a workout by time if I'm at a pool with no timer). The Garmin also counts down the reps left in the workout, which is very nice (it's surprisingly easy to lose track when you're pushing very hard).
Some people complain that the Garmin is too big, but I've never had any discomfort from it, and my wrists are tiny by almost anyone's standards. The one hitch is that the Garmin's heartrate monitor does not function underwater; to do HR training, one apparently needs to invest in a Polar HR monitor.
H20 Audio ipod case. When you're cranking out a lot of hours in the pool, having something to entertain you is key. The nice thing about this case, as compared to others, is that it allows you to control the iPod while it is in the case (as opposed to others I've seen or borrowed, where you need to start the iPod prior to placement in the case, and are unable to change anything once it's in the case).
Depending on your bathing suit, you may need to have an aquabelt in order to use this case. The case has a clip, but a woman's one piece bathing suit lacks a good place for the case to clip onto, requiring the use of a belt in order to have something to clip the case to. (men's swimming trunks or a female two piece won't have this same issue). I also note that the case doesn't float, and so if it does become unclipped, it may very well fall to the bottom of the pool. I work around this by looping a shoelace through the clip, and then through my aquabelt, as a safety of sorts.
Aqua jogging belt:
There are people who run with the belt, and people who run without. There are people who swear that you can't get a good workout unless you run without the belt, and people who swear that they need the belt.
For myself, after a lot of work, I've developed the ability to poolrun without the belt, but have also noted that I don't seem to get a good workout without the belt -- I focus on staying afloat, rather than on actually pushing the workout. So, I run with the belt.
Most pools have belts for use. I pool-run at several different pools, and so have test-driven several different belts. I've developed a preference for the Kiefer belt. It's smaller than other models, and so less encumbering. It's also pretty soft, with a stretch belt, that makes it much more comfortable than other belts. The only thing I can't speak to is its durability.
I don't have these yet, but I want them. Supposedly they increase the water's resistance, adding to your workout. For me, the clear benefit would be in increasing the resistance as you push against the water with your foot, making the pool running slightly closer to real running. I do note that many pools have signs banning these, though I don't know how strictly that ban is enforced.
The AQX model seems to be the model of choice, though it is also sadly discontinued.