Subject: Are subs overpowered?

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Me and friends started playing maxr. I always though of subs as being overpowered in original game, and it seems its the case here. Now, the question is: are they really, or simply we play the game not good enough?
The thing is, when one player builds shipyard enough turns before any other can, he can build subs and prevent other players from building anything on water. We tried it a couple of times and it worked always. Subs are cheap, and except ground attack plane i think there is no other unit capable of destroying them (if the player hasnt built his own shipyard). And except corvette - there is no unit capabe of detecting subs? This means the player using subs can hunt for any constructors/ engineers that are in water, surface and attack. Even if the other player has ground attack planes to destroy those subs, it will be an exchange of casaulties. Sub for engineer/constructor. Considering that eng/const would be loaded with materials, this pays of for the sub owner. It prevents the defending player from building fleet, bridges and accesing minerals on water. Gives the sub owner control over water... So, how can this be countered?
If there is no good counter, i though of adding sonar like detection to scanner air planes ("awac" was it?). There is xml file with units, would copying value from corvette's xml file into awac's file with 'detect stealth units' work?