best way to track an open water swim?

the challenge is, that water absorbs GPS signals.  here are the three ways I can think of.

1.  Pretty much any GPS watch that is waterproof- or even water resistant... and stick it in your bathing cap.  I've had very good luck with this.  tip:  set it to autolap every 1/2 mile- so you can hear it chirp and know how far you've swam. 

2.  A specialized GPS watch that can do OWS while on your wrist.  There are only a few models that will do this.  Fenix 2, Ambit 2, 910xt. And even they aren't quite as accurate as the bathing cap trick.  it's no simple challenge for these watches to capture a decent signal while being dunked.

3.  Waterproof case for your iphone and put it in your bathing cap (if that would even work?).  If not, i guess you could drag a buoy behind you with your phone on it?  (not really practical- but certainly possible)

my guess is that if you OWS and you also pool swim - you would enjoy a proper gizmo.  

There are many fine choices out there, each with their own pro's and con's and individual features.  I'm a little hot on the Ambit series these days, given how powerful, flexible and STABLE they are.  Plus- for $219 - this is a screaming deal for this much capability:  http://www.clevertraining.com/p-8113-suunto-ambit2-s-sport-gps-watch.aspx