As far as I remember the rule states "No CLOSER than 500 feet from any person, place vessel or structure".
So in theory you could fly alongside the yacht 500 feet laterally from it, 2 feet off the sea. However, the risks are pretty high...gust of wind, seagull flying past the propellor - I know both of these that have happened to people at my flying club. They got away with it...this time!
One 'tip' from an old instructor of mine. If you do want to fly low....never low & slow...do it with a lot of speed and plenty of up trim.
The best 'low' flying for the sensation of speed is skimming the top of a cloud...and provided you've got an IMC, that's legal.
I'll wait for a good telling off from you guys now for suggesting this is possible!
