Yes, Martin - you've got the idea. I liken it to orienteering on water, going from checkpoint to checkpoint and being timed between. Perhaps even a bit like rallying. Martin's suggestion of a venue around the Thames estuary which has open water, islands and creeks is a good one. And there is our local infrastucture in that area to help make it happen.
To see the sort of thing type 'archipelago raid' into google for pics., video, etc.
You can drink as much beer as you like, Derek. I don't think there is any legal sanctions for being drunk in charge of a Sprint 15 on the water
but you may wish to save your thirst for when you have reached land at the end of the day