I am a little uncomfortable about this single destroyer/submarine unit idea.
When I play most great marintime nations (Japan, USA, UK, also Italy and Canada) in vanilla, I often build an enormous number of destroyer flotillas - frequently in excess of 100, sometimes 200 - for reasons that range from stopping subs, to protecting high-value assets, to blockading enemy shores and owning the sea, to cheaply building up a large force and thus promoting all kinds of leaders (not just admirals, but army and airforce generals as well).
Now, if CORE makes destroyers single units, and thus (I suppose) each individual one cheaper, then each of the great marintime power could easily wind up with even more individual destroyer units, which will impose heavier CPU costs and requires more player managing. Changes to naval combat stacking modifiers, fine-tuning not merely strengths and costs but also visibility and sighting, a clear understanding of how players can use massed units to blockade, watch, own, and patrol - all have to be done if you go over to a single-unit approach. Similar comments apply to subs.
