...Depth sounder showed 24 feet of water and my bow hit a rock ...My keel didn't touch bottom as there was lots of depth right up to the rock.
Bad event, sorry to hear it. When I hear depth sounders recommended I foresee the type of event you experienced. We have loads of rocks and ledge rising from deep water. A depth sounder would be great on a shelving bottom, but it's the rocks that scare me. I've almost hit uncharted rocks in Moosehead Lake, which is poorly charted.
Casco Bay just north of Portland is well charted, heavy ship traffic during the war, many areas wire dragged for rocks, etc. I had this year's Bluechart on the Garmin, so it was recent. The GPS had a clear view of a clear sky, and time to correct as I approached. Electronics are great to have, but not infallible.
I guess we need forward looking infra red radar, or forward looking sonar under the hull!
Two weeks ago I penciled in an uncharted rock in a narrow but beautiful and popular anchorage. It was off to the side, but I like to tack up to where I anchor and might need that space to get around other boats. I'd gone around in the dinghy at low tide and looked for hidden problems, as I hope to go back there sometime.