The best swim training aid is a relatively safe, clean body of water. :-) Other than that, pull bouy, fins and paddles are also good.

I'm no expert on swim coaching but if someone that I was coaching showed up with a swimP3 or the underwater ipod gizmo I'd tell them to leave it in the bag. Too many things to concentrate on while swimming to have proper form and etc to be worrying about what song is coming up next or if the earbuds are falling out and what not.