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.
I was building an underwater mapping GUI, a quick SCUBA dive to do some testing before the weather closed in whilst at sea in the GBR Australia.
Dropped to 10 meters, set the datum and started swimming, going well until I crossed into a negative quadrant. My beautiful track went haywire/dead.
Then I swam around trying to get back "into" the positive quadrant, I couldn't find it and consequently couldn't get back to the datum. When I surfaced I couldn't see the dive boat!
After some time contemplating my life/coding choices for literally jumping in without thorough testing, I heard the Zodiac inflatable buzzing around the rising waves looking for me.
Back on the boat I discovered that I had only coded it to work in the positive axis from the datum. In the other axis the location was undefined.