i think Mirelle actually found his way by using the water temperature, knowing that deeper water is colder that shallow, and drawing a nice graph by calibrating the temprature curves at various times of the year using the lead line. Possibly...
IF you tie the unit onto the end of a piece of line with markings on it, drop it over the side until it stops. Then pull the line back in and see how much of it is wet, you can calculate the depth.
Thus the sounder has still done its job, ie worked, to help you find out how deep the water is.
profound question, indeed. Do ducks quack, is the pope a catholic? each indicates a functional relationship one to the other eg would a pope quack or a duck be a catholic? in a purely literal sense one can have an agnostic depth sounder which works in the sense that it is in a state of working potential but has yet to reach that ineluctable and divine state whence electrons fire together to momentarily produce a desirable result eg 5.2m at puilldobrhain as if experiencing a rare orgasm ...
We say pah to depthsounders, get a fishfinder. Contours of bottom look great in colour and the trend to deep or shallow much easier to follow. Oh and it shows fish as well.
.....in a sort of way. Fish with swim bladders show up as little arcs like the top of an umbrella, bit like a crooked finger. Captain Birdseye shows up as a prat.
Oh God - you can't buy portable ones can you - i can just see the old scrote turning up with one when he comes on the Distressed Gentlefolks Summer Cruise
Now lookee here, that is a very unfair stereotype of a fisherman. It is a bit like saying that all yotties speak with a Scottish accent and sail Bendietoys.
BTW, is your Etap one of those with a lifting keel, a bit like the one that was aground for six hours in Keyhaven last Saturday?