I had a thought, having played this rule set quite a number of times now, that maybe it would be more in line with the concept of submarines if you could not hunt a submarine with another submarine without a destroyer present?
This would, of course, make some fundamental changes. For one, if you attacked a submarine with a submarine and destroyer, it would only be possible for the defending submarine to attack the destroyer in opening fire because it could not attack the attacking submarine until it decides to stay (and thus become automatically detected.)
But it would also allow you to put your submarines in CR range and force your opponent to deal with the loss of income or build more expensive destroyers instead of just dropping half a dozen submarines and forcing you to retreat or attack them.
Just an idea.