David,
I still, vividly recall, the S61 incident of 1983, I think it happened on my birthday, a Brymon Twotter, the first of the day, had just managed to land in ISC after the fog had lifted a little, as the S61 was reported overdue/missing the Twotter got airborne, leaving pax on the ground, for search and rescue purposes, the pilot took it upon himself to do that.
Oostende Lines had their 3 (4 including the reserve) ferries also operating the same route as the jetfoil, in rough seas they would cancel the jetfoil for passenger comfort (vomiting) reasons and without any commercial pressure to operate, they simply put the jetfoil pax on the next ferry saving on jetfoil operating costs.
Indeed the first time I travelled on a jetfoil were in somewahat rough seas, in thiose conditions they took the shortest route from Ramsgate straight across the channel to the French Coast then we 'flew' along the French/Belgian coast, just off the beach, to Ostend. Rough seas, what nonsense, throughout my life I've had such a problem with my stomach and on that occasion I didn't even spill my gin and tonic, I was really enjoying the experience!
But I read on tripadvisor that the Scillies ferry is also reknowned for vomiting
How about a Griffon Hovercraft?
Several different sizes available, anything from 20 passenger to 180 passenger craft, as an example the 54-82 seater cruises at 45 knots,a recommended maximum wave height of 1.8m and using 230 litres of fuel (at max payload of 9.3 tonnes) per hour, that'll overcome the shallow water problem!