Nik, I think I'd be looking at an engine housing which keeps the salt water out
I am not aware of saltwater getting in. Overall, the ride is pretty dry. The corroded part is wrapped in that silver heat resisting shroud. Also, I would of thought that the turbo is so hot that any water getting on it would evaporate immediately.
I can only assume that it is a combination of heat, (look at exhaust manifolds), and damp air.
PS. If you look at the pictures, the exhaust side is corroded, but the boost side side is near pristine