Clive,
I spend a huge amount of my time telling people to slow down including Richardsons boats, when its bad enough to stop the hirer and speak with them, they usually they have a rev counter, which is usually calibrated incorrectly.
So I fill in a report, stateing the problem, a copy goes to the hire firm.
Then the following week the same boat comes out, has it been recalibrated, probably not.
So why not put on a GPS, how much extra would it cost on top of a hundred thousand pound build, problem solved.
Then when the on the spot fine system comes in, your customers will never be stopped, or if they are, there will be no excuse.