I don't know about the rest of the world, but here in UK our "copper" currency is no longer copper, its copper plated steel. Many coin op machines, as part of their coin recognition process, use the magnetic signature of the coin, and one that has been magnetised might be a source of annoyance if you couldn't get your drink can from the machine, or, possibly, your car out of the car park.
If the magnet can be retained on the boat, no problem. If not, it would need some sort of housing maybe like a retractable USB stick to put on a key ring. I think that they could be a great device, but, like anything new, there may be some minor pitfalls to avoid with a bit of thinking through.
Edit later today. Did a bit of digging. More information here - http://planet-soaring.blogspot.co.uk/2012/02/magnetic-switches-become-more-and-more.html
and here - http://zepsus.com/smart-magnet-holder/ and a separate magnet is seen as a problem that can be solved.
It does appear that one pass of the magnet toggles it on, the next pass toggles it off, but no real clues yet as to whether it is direction sensitive. Could be the way to do things in future, especially for small models.