I lost a rudder in the silt under my dock. My diver buddy went down, wiggled it, so I thought he had it, I pulled the pin... WOOOSH depth charge away!
He looked but couldn't find it. So he ended up dropping a long line off the stern with a weight on it. So the line went from the boat, to the bottom with a weight, and then there was like 10-15' of extra rope down there. Then he started searching in circles using the rope as a guide. Search right around the weight, then slowly work your way out using the rope as a guide. He found it that way.
Depending on how deep the water is, you might need more then 10-15'. It might plane a bit and be farther away then you might think.