If you wanna drive your car underwater, get a diesel engine. The whole design is much better suited for waterproofing. Most modern gasoline powered cars have way too many electronics to be reliable under water, and it would cost a lot of $$$ to try and modify something that just wasn't designed to be operated underwater. There's a lot more to it than just slapping a snorkel on the intake.

I don't know where you got the idea that the Land rovers have a waterproof engine and passenger comparment, the ones I have known had trouble keeping out a heavy fog. I know they can be made fairly water resistant with the right modifications. Also, driving through deep water is more than just keeping the engine running. Just don't try and drive your new stock SUV through a river unless you have a very good insurance policy and a lifejacket.