I would not presume to give you advice, Donna, since I do not know your exact situation. I can, however, tell you what I would do in a similar situation, and perhaps it will give you an idea or two.
From what you have said, it dropped in about 5 feet of water, the bottom there is muck, and the visibility is very poor. However, you were in your slip, and presuming no strong currents it most likely went pretty much straight down. Arrowed, as you said. Keep in mind, though, that it most likely went off at a bit of a slant, because that is the way things do in the water.
So it is down there, you know that, and most muck only goes a few feet at most, with firm bottom under that.
So, if I were in that situation, (and I have the advantage of being a diver, which presumably you or your other half are not), I would lay out a large grid on the bottom, using weights and lines, to not only outline the area where it most likely is, but also to enable a methodical search pattern.
Then I would put on my scuba gear and go down and just lay on the bottom. I would begin at the top left-hand corner of the grid, and go back and forth with my hand, staying in straight lines, until I got to the bottom right-hand corner of the grid.
And probably I would find it. I have done similar things with eyeglasses, and they are much smaller than your aluminum piece.
Anyway . . . I submit this just for what it is worth. Doesn't take much diving skill to do that, and your local dive shop could probably recommend a kid who would do it for peanuts, or even just for the fun of it. (May not sound like fun, but it is a challenge, and a challenge is fun).
