See, I'm not looking at this as an "I'm macho, I don't need it" thing. It looks to me like more of a safety hazard than a preventative.

Here is typical equipment used for tethers.
http://www.apsltd.com/Tree/d312000/e310335.asp

Note that on big boats the tether is intened to never allow you to leave the boat. You should be tied such that you can't make it all the way to the water. On our boats we're required to get out beyond the hull, hence typical tether mentality does not work here.

So... say Robbie was tethered... but the boat didn't flip. How does that story end?