I've rendered a C3 disabled and afloat and she's taking forever to sink.

I would finish her off with the deck gun but the weather won't allow me and I don't want to waste another torpedo.
My question is, can I mark the exact position on the map, continue patrolling (heading for the first time to loch ewe) and return in favorable weather to sink her with the deck gun? I imagine the game does not render rescues/tows from allied ships but will it stay there? If it is gone I can check the depths to see if it eventually went down on its own but getting no credit for the kill would be disappointing..
Anyone else been in this situation?