[nate_meurer]

The argument you heard is correct. The casings are likely made of something that is completely unaffected by saltwater, the pressures involved are neither astounding nor even interesting in this context, and... marine organisms? Which ones? Harsher than a botched re-entry?

[madaxe]

The casings are steel, the rtg is in a ceramic capsule. You'd be amazed at what barnacles and such can pry wide open given time.