I posit that open water swimming and in most cases outdoor pool swimming would would have fewer issues since the spaces we swim in are generally open areas. Indoor pools are obviously off limits.
Regarding a race director who gets nasty emails by competitors who accuse the director of an inaccurate course due to their own personal GPS data had this to say...
From the New York Times:Note, that Mr. Helton runs is in an area that gets great satellite reception, no real trees or buildings in the way, hence his accurate readings. Open water, especially in the ocean which GPS was originally designed for, would probably be the most accurate use of GPS. I place strict emphasis on the word 'probably' for your mileage may vary... Quite literally.
Mr. Thurston has gotten so used to the complaints that he actually has a generic e-mail reply. No, it says, the course was not wrong. Your GPS device was.
“If someone wants to go to mat on it, I ask them to go to a 400-meter track and run on the inside lane for 12.5 laps. That’s 5,000 meters,” he said in an interview. Then, he tells the runner, check the distance on your GPS device. He guarantees it will not be 5,000 meters.
[Later in the article] But Mr. Helton is not throwing his watches away. He has three GPS watches and uses one nearly every time he runs. Then again, unlike most areas where I run, his routes do not have large trees, winding roads and poor satellite reception. On his routes, Mr. Helton said, his GPS device is accurate to within 10 feet of where he actually is.
The article is a must read if you are considering a GPS. Lots of observations and how the systems work and I really want to emphasize that GPS systems for swimming is a real option, but they these systems like vectors not sine curves; (straight lines versus s-cures), and should really be considered.
UPDATE: From Rob at RobAquatics - Boy did his mileage vary. See below photo:
He had this to say in the comments section: "I've found mine to be pretty good in open water, but not the pool. I did a test in a 50 meter pool and got some pretty creative results."