From what I’ve seen some insurers have an exclusion about being driven deliberately into deep water. Otherwise they might go with a “causing deliberate damage” exclusion.
But it’s generally subject to a careless/reckless assessment – so should the driver have taken precautions. Like looking at the signs, stopping, slowing down, taking another route, driving according to the highway code, etc.
Or did the driver go into the water unknowingly.
If it’s reckless, the claim would likely be declined. If it’s careless then hopefully it’ll get paid. Evidence is going to be very important here, but frankly given the staggering levels of buffoonery (and social media coverage) at Rufford I doubt many insurers will be looking to fork out willy nilly for those claims.