If docks are present, try positioning your boat out off the deep end....say, one cast length away. Concentrate mostly on the deeper dock poles and/or floating docks. Sometimes the fish will suspend, sometimes they'll be close to the bottom. Feel for brush off off those dock ends. Try a texas rig, jig, or Senko type weightless worm. Also, steeper shoreline banks are indicative of deeper water as opposed to flatter ones.
Until you get some type of depth finder, it will be a slow process, but it can be done.