A side scan sonar would see it quite easily. The resolution on them is amazing. The higher frequency sonars can tell what model of car that is sitting on the bottom. Side scans are tens of thousands of dollars and it takes special training to be able to interpolate the image on the screen
. We have used them from the research
vessel to find all kinds of things. The more practical alternative is to rent one and the person who is trained in using it. This would cost you 2-3 thousand per day....possibly more than the value of the mast
A magnetometer would never detect a stainless and aluminum
mast, for obvious reasons. We use those as well. A company out of silicon valley charters our boat to field test their magnetometers and sub-bottom acoustic profilers. The latter are used for seeing the Earth's strata.
With an underwater camera
you might get lucky. It sound like the water
in your marina is too turbid to be able to see more than a meter or two, otherwise you would be able to see the mast sitting in 9 feet of water
. This makes a camera a needle in a haystack situation....the greater the turbidity, the bigger the haystack. For a camera to work in that turbidity it would have to be flown a very precise 1 meter or so above the bottom in order to see the bottom otherwise all you would see on the monitor
is solid green. Doing this would be nearly impossible without a camera mounted to an instrument with controllable diving
planes and an altimeter. In doing this you also take the chance of running the camera into the mast or some other junk on the bottom....not good.
A recording depth sounder
would probably not work either because you would never be able to tell the mast from a fish's air bladder sitting at the bottom from any other sort of debris at the bottom.
The best inexpensive way would be to do transects dragging a grapnel hook by setting up a grid on your electronic chart. We have done this as well for recovering lost
instruments. We usually find them...knock on wood.
We had a couple custom grapnels made because what the boating
supply stores were selling was inadequate. With the wide rather than circular (cross section) flukes the anchor
would tend to grab rather than plow. Which is understandable since that is an anchors job.
The grapnel for boats would hop along the bottom possibly missing what we are looking for rather than plowing along the bottom.