Both surface ships and subs are involved.
Cards provide the detection levels or the detection countermeasures.
The higher the detection level the better and harder the attacks become. If a sub detects a surface ships he's the only one who has the detection marker raised. If ANY member of a surface group detects a sub ALL members of the surface group raise their detection levels on the sub. This one isn't quite as fast and furious as GMT's Rise of the Luftwaffe but you can run several battles in an evening. Sub is a card game.