[CCIP]

I think it speaks volumes about the viability of a surface commerce raider that most nations wouldn't even consider a diesel submarine viable as a raider on the high seas, at least against a sophisticated enough foe. Having to snorkel every once in a while is already considered enough of a liability for detection on the high seas... That and the inability to get out of an area at high speed underwater really do compromise even diesel subs, unless their raiding grounds are narrow, shallow seas with lots of places to hide.

Another idea: a large, well-equipped, stealthy (VTOL?) seaplane - or maybe even an 'ekranoplan'/WIG aircraft may well be a viable commerce raider. Again, it'd probably be a platform that would stand off, launching fire support and boarding parties from afar, but it could be a way to target and capture ships on the high seas before any sort of help can arrive. Actually, I think that could have been a role well-suited to the Caspian Sea Monster, assuming it could sortie undetected or loiter somewhere far at sea...