A depth sounder shows one number, but it's only useful if you know where the measurement starts and how much bottom it covers. Most units that "don't work" have a mounting or calibration problem.
The transducer sends a pulse and times the return. The unit converts that to depth using the speed of sound in water, roughly 1,500 m/s, which rises slightly with salinity and falls with cold. That is why the same spot reads a little different in August and January โ normal, not a fault.
Then the real question: is the reading from the transducer face or from the waterline? If the unit is set to depth below the transducer, add your draft before you judge anything. In two or three metres of water around Kuwait that difference is the gap between a clean pass and a grounded keel.
The face needs clean water, clear of the prop, strakes and anything that makes bubbles. Mount it level, or with a very slight tilt aft. Too much forward tilt throws the beam ahead of the boat, so you read the bottom in front of you and assume it is the bottom under you.
The beam is a cone, not a line. At 200 kHz it is narrow; at 50 or 83 kHz it opens up wide. In 10 m of water a narrow cone covers maybe 3.5 m across, so a fish holding two metres to the side never appears. Don't call a mark empty after one pass.
Tie up where you know the depth โ a berth or jetty with a marked chart figure โ and compare. If the error is constant, set the offset once and stop thinking about it. Then confirm units and the salinity setting, and put gain on manual: auto gain hides small fish sitting tight on the bottom.
Kuwait's inshore is shallow, warm and full of distractions. Bubbles from a wake or a turning prop read as bottom. Weed gives a soft, drifting return that looks like structure. Hard rock returns early, so the number comes out shallower than reality, while soft mud lets the pulse in and reads slightly deep. Above about 15 knots the return breaks up and you get noise instead of a bottom trace.
Don't run straight lines over a mark. Work a zigzag across the edge and note where depth falls two metres in ten seconds โ that is a drop-off or a reef edge, and fish hold on the edge, not in the middle. Depth also changes with the tide, so check Shuaikh and Shuaiba tides in the B7R app before you fix a pass time, and save the spot with the marine tools. Log the depth and position in the catch log afterwards and within a few trips you have your own edge map.
All of that sits in the B7R app: Shuaikh and Shuaiba tides, sea state, fish guide, catch log, seasons and marine tools โ free on Google Play. Install it before your next trip. https://play.google.com/store/apps/details?id=com.q8dm.b7r