I like the idea of subs being low-HP high damage units, but the dynamics of water play are not like land Vs Air.
High damage subs that could push an enemy navy out sounds good on paper, because most subs can not effectively do anything about land. The hard counter to subs would be torpedo bombers. The counter to that being cruisers. Under this dynamic, a Seraphim navy would be pretty dangerous, sub rush to push the enemy navy out, cruisers to shoot down torpedo bombers, and long range tactical missiles to bombard land targets. Of course, the exchange rate of using suicide torpedo bombers against cruisers is better for torpedo bombers, torpedo bombers are cheaper.
If torpedo turrets had more HP or torpedo defense, it could change the dynamic of high-damage subs pushing someone out of the water, at least until destroyers show up and out-range the torpedo turrets.
The dynamic of water is interestingly complex, but I would like to see subs have a clear purpose.