With all the posts on this thread, I'm glad I split it away from the Tackle and Equipment thread it was on.
I found the difference in water depth was easy enough from the bank WITHOUT a fish finder, in fact probably easier, as mapping by hand (if anyone ever saw my Earith map @elmoputney ) and with a marker float you have bankside references or aiming points, whereas with the Deeper you would still have to reference it in note form.
The high and low water points can be quite off putting. I think Earith lost a metre of water depth, and Alton was 3 or 4metres, and you could gain an extra 10metres of bank as the water dropped, however some features that produced fish in full reservoir times, could also produce in low water times, yet some were totally above water when the water was low.
A feature that produced fish when it was 8ft deep, (marker float counting hence in feet) still produced when it was 20ft deep.
I mention cross references in note form, on the Deeper can you put in GPS positions or does it have to be additional notes?
Obviously reason I ask is that sea going boats do have regular fishing marks that captains use and fish regularly, and they would have to be GPS.