I don't know if this has been suggested before, but I think that subs should be able to pillage sea tiles when you're not at war in the same way that spies can sabotage land tiles. If no-one can see your sub you should be able to pillage with impunity. However, if a destroyer or sub is within range your sub could be spotted in the same way that your spy can be caught. If spotted the other civ would have the option of how to respond to your sabotage and you could get a -1, again just the same as with a spy. I'd even accept having to have a spy on board a sub in order for it to commit an act of sabotage.
It's frustrating being able to deny oil to my opponents on land, whilst being unable to do anything about their oil platforms without declaring war. If they get annoyed with my sabotage and declare war, so be it.
It's frustrating being able to deny oil to my opponents on land, whilst being unable to do anything about their oil platforms without declaring war. If they get annoyed with my sabotage and declare war, so be it.