First of all I respect you for your commitment to the game and for Larry, I’m a big fan. The rules for subs are vague, indeed. Personally, I feel you have set a rule here that has a tremendous impact on all naval operations for every country with a boat. A cheap group of subs leads to a massive escalation of defending destroyers and subs. This is power.
To say, just buy one Destroyer is not enough. A fleet of subs are capable of irradicating a surface group even with a destroyer, seeing that they can only hit surface units. So, who cares if I lose 6 subs(36 ipc) if I can take down a surface group worth 60+ ipcs. The only way to avoid this is to escalate more destroyers per surface group, or buy subs myself, to take hits?
Now I should attack that fleet of subs, with the advantage of having a Destroyer, but that requires at least a second Destroyer to stay home with my surface group in case any subs survive.
This is how my boards are looking, in both the Atlantic and Pacific. And under these rules, even with a Destroyer, Subs are king of the sea.
Some suggestions -
Return subs to their status of being “exposed” when attacking even without a Destroyer so the surface group can defend themselves.
Reduce subs to a one roll attack/defense and force submerge at the start of Round 2, unless they are defending with an attacking Destroyer present.
These make a sub fleet strong still, but not a cheap and easy underwater game changer.