Comment by mschuster91

The powertrain is one thing, the more critical issue is the car's structure, including the ventilation system, all sorts of gaps - and also, all hollow spaces, in which you need to balance weep holes (to prevent water condensation and subsequent rusting or weird issues regarding temperature changes) against the ability for external water to end up there at all.

Getting that right is a very expensive job and that's why you usually only see true (i.e. no visit to a shop needed afterwards) wade ability on large military vehicles and custom RV builds.