seaside rust - does it ruin underneath?

I came across a Cadillac Catera (rebadged Opel) with only 30,500 miles on it. 2 owners (same family) and unmodified. Only catch is that it has been watching sunset over the ocean for many years and is rusted beyond repair. Interior is still excellent condition, so you could at least strip it out to fix up another car. The alloy wheels are rather pitted too. Now what I am wondering is: does the salt air get underneath and attack everything else. Would engine and transmission be okay?