The thing about insurance, especially if your gonna try and 'get away with it a bit' is to not take the piss - it has to be plausible.
Anyone could look at your marshan and know it goes over 50. RA Carter, I think, will insure pretty much any speed, as long as size is under 25 feet or so. RA Carter also offer good levels of cover for skiing, towing toys etc.
There are others also - Porthcawl springs to mind.
I visited craftinsure at the boatshow a while back - they take a really grim view on anything fast.