You are looking to fill a need that is pretty tough based on our experiences of the principles/physics behind how the FLS works. Image you are travelling through an area 10' deep and 100' away it shallows to 7'. In order to get this kind of detail on the display you need to bump up the gain and suddenly water clarity, bottom structures, bottom make-up, currents and even freshwater streams affect the sonar signal and it might look like you're travelling through a snowstorm. But, having said that, we've used our FLS set to look ahead 100' and have readily read the depths in the 15 - 20' range
On our sailboat we have the Interphase ColorTwinscope and would never own another boat without it! (No we don't work for them or get paid for this, it is a good product that performs well!)
This is our third summer cruising Alaska and Glacier Bay. During this time we've literally relied on the FLS to get us into or out of areas that we wouldn't even looked at without it. The Twinscope offers two modes Forward Scan or Side Scan. Looking forward is like tilting your regular sonar at an angle and shooting it ahead of you; Side Scan projects a 90° beam (45° on each port and starboard) showing you the sides of a channel.
During the winter of 2008-2009 we spent the entire winter cruising and exploring Glacier Bay, Alaska. The FLS was useful in looking ahead to see submerged icebergs (see here
), uncharted hazards, etc. In Glacier Bay, Adam's Inlet is closed to vessel traffic during the summer months, however during the winter it is open. The entrance however runs fast during tide and can really only be traversed during slack. Even at high tide there are areas that are less than 8' deep. We used the Interphase to track the bottom and found the clear path to enter the Inlet (see here
) . Once inside it was an unspoiled beauty seldom seem from the cockpit of a sailboat. It was rewarding!
Like all things it's a trade off, but it was a worth while investment for us.
Otherwise, consider putting the youngest crew member in a scuba suit with lead boots 100' ahead of you reporting back...