I think this lends itself to being a ride with indoor sections/an entirely indoor dark ride. Given that as of this year we have nothing like that in the UK (until Valhalla re-opens with the cost cutting measures implemented), it's something that could easily do well in our climate.
With regards to the Merlin H&S comment, they've been overly anal about everything they do for some years now; I've seen it from both a guest and staff perspective. When I've jokingly shown H&S responsible people within Merlin videos or images of balls-to-the-wall rides or concepts, they've looked horrified at the thought. Not because they personally are shocked, but it's the way the militantly strict H&S standard within the company has honed them to be. It's a consensus in everything they do, not just an approach to water rides.
This concept could be signed off by every single HSE under the sun (after-all, Mack wouldn't even offer it otherwise), but I still don't think Merlin will touch anything like that with a barge pole.