Here's how I have been doing it. If you have a ship with radar, roll for each enemy ship on the table. If you acquire and ID the ship (ie pass the roll) the ship is locked on radar for the rest of the game, unless you are Germans, in which case you loose lock once the ship is out of your front arc. If you fail to lock a ship, roll again next turn. Although not in the rules, I think it should be such that if a sub is locked and then dives, lock should be lost. You can attempt to re-acquire once it pops up, but you can't detect submerged targets with radar. Like I said, that last part is not in the rules, but I think it would make a good addition.
BS