Not much. A floating GPS antenna on a self tensioner, tablet in a housing. And some buggy code :) Good enough for swim lane mapping and preventing holidays (unsearched areas). Another system we worked with used ultrasonic positioning from beacons, very accurate and covert.

Great story and it made me realize that usage of "holiday" is more general than how I thought of it (missing a spot when painting). Which led me to http://www.word-detective.com/2010/12/holiday/