if felt sonar was too expensive way back when it was 75(?) metal for slightly-below-radar range. this was when there was less surface to sub interaction tho, and urchins lacked sonar afaik, so the necessity to have a sonar was greater then.
still i feel sonar should not be significantly more expensive than radar, and it probably would be easiest to just have it integrated in sea-built radar stations.
i dont see the whole concept add anything but a nuisance to the game. finding units underwater is like poking in the dark, especially on vast water areas. you may or may not stumble into your opponents underwater units, you dont really have much control over it.