The heat exchange is between air and water. And that water happens to be seawater in this case. Seawater is corrosive. Not to mention that if your inter/aftercooler leaks, that seawater goes straight into the intake of your engine.
On this subject, the vast majority of boat owners with high powered turbocharged diesels don't know that aftercoolers require service. A service that can prevent the above from happening. There are boat owners, maybe the B&Bs of the world, that have their engines "serviced" and have no idea what is being done. I mean no disrespect there. My point being you do get into a realm of "managed" boat service where the owner does not know every little detail. So turbodiesels do require a little more vigilance than land based turbo applications and if that vigilance is neglected, the consequences could be severe!!!