bearface,
The waters around the Scillies are so shallow the Scillonian ius flat bottomed which causes passengers to vomit ... regularly ... and having, this morning, taken a wooden outrigger between Dapa and Surigao with the waves coming over the top of us ... I have every sympathy.
For fear of commercially advertising one can check out the specs, including craft sizes and acceptable wave heights, by visiting such a site as
Products - Griffon Hoverwork
As for DHC7's, these remain out of production for some 25 years past, there are none on the British register and were a solitary DHC7 to appear on the British register then it might work an ISC route for perhaps 2 days per week during each summer and sit on it's bum each winter whilst other operators struggle to make a once a day Twotter or Chopper pay for itself ... and as for any DHC7 spares back-up ... well go figure.