Are we talking about using it while cruising, to scope out anchorages? Where you only really care about 5 to 20 feet?
Or are we talking about using in open water with a hundred fathom line while underway? Where you have tallow in a recess in the weight so you can sample the bottom as being sand, gravel, or mud?
If the latter, you're on your own. As for while cruising, it's pretty simple. I used to carry a couple of them aboard, one with a 12' line in the dinghy, and another with a 30' line for the mother ship. Rather than knots I used strips of tyvek marked with a sharpie inserted through the three strand every foot up to 10', and every two feet after that. Using it was as simple as dropping it overboard until the line went slack, and noting what mark was close to the surface. BUT, doing it while underway, even at just a couple of knots, takes some practice. Be sure to have a loop in the end of the line wrapped around your wrist to avoid losing the line altogether (you can imagine how I know this last part).
I think it's actually a very useful thing to have aboard. In particular, we were frequently anchoring close to shore where there was a steep drop off. If the boat swung to shore, the depth reading at the transducer did not reflect the depth at the rudder. If I were worried I would drop my lead line off the stern to see how much I had.
And I used to use the one in the dinghy all the time when scoping out new anchorages. That is until my wife bought me one of those depth mate hand held depth sounder units, which are GREAT fun to own.