Apparently, I am so inept at keeping track of pool laps that not one but TWO of my loved ones decided to help me out with this problem over the holidays. Or, they were just tired of hearing me complain about said inability, or listening to me refer to workouts as "3200 yds plus or minus 200 yds".
I've started out with the Poolmate watch by Swimovate. I've swum for about 3 weeks with it now, and finally have it figured out. For the record, if you read the instructions manual thoroughly first, it won't take you 3 weeks to do this. If you're a triathlete excited about a new toy and don't sit down and read it first, it'll take some experimental button pressing.
In all my workouts, it has accurately counted all my swim laps - no misses! The only downside is that it doesn't count kicking sets, since it tracks the stroking movement of your arm. So front kicking, back kicking, side kicking, no counts; it picks up again when I hit 6-beat switch drills.
It also stores your workouts, and lets you scroll through average stroke counts and efficiency (measured swim golf style), total distance (yds or meters), time, and number of laps. Cool.
As an interesting experiment, I've also worn it while deep water running - wondering if the arm pumping motion (I do travel back and forth across the deep end significantly) would pick up as "laps". The answer is no. But I don't really want to know how many mindless back-and-forth lengths of a deep end I have "run" anyway :)