I don't think there's a difference between driving for 30 minutes and driving for 4 hours - in both cases you're at operating temperature.
Personally before entering a water crossing I'll either have driven very gently to let the engine/turbo/brakes cool as much as possible before the water hits, or I'll wait with the engine idling so that things are able to cool for a couple of minutes.
That said, even a hard run into some water should only impact on the turbocharger housing and the disc rotors.
I'd call warranty on this one.