As far as measuring depth, can you use a slip float and some weight? Cast out to where you want to take a measurement, and allow the weight to pull slack line through the float until the 'stop' makes contact and pulls the float into a loaded position. Repeat this process, adjusting the depth stop to a little deeper setting each time, until the weight contacts the bottom before it pulls the float upright. Measure the distance between the stop and the weight, and you'll have a pretty good idea of the depth.
