fdarden has made some very good points. Maybe it goes without saying, but the first item of business after following the anchor line to the bottom is to manually set the anchor!
But back to the original question - when I was much younger, my friend and I routinely dove from his unattended boat. The dangers of this hit home one afternoon when we surfaced about 50 yards from the boat. Once back on board, we suddenly realized that we were about a half mile down coast from where we anchored to start the dive. Fortunately, we and boat drifted at about the same speed. This could have had a much worse ending.
Now that I have my own boat, there is no way I would dive with it unattended. Plus, I make sure the surface person knows how to operate the engine controls and radio - just in case. I also dive with a live boat quite a bit, which means I don't anchor, but leave the engine running and the surface person follows our bubbles at a discrete distance. Don't have to backtrack at all and the boat is always close in an emergency.
But back to the original question - when I was much younger, my friend and I routinely dove from his unattended boat. The dangers of this hit home one afternoon when we surfaced about 50 yards from the boat. Once back on board, we suddenly realized that we were about a half mile down coast from where we anchored to start the dive. Fortunately, we and boat drifted at about the same speed. This could have had a much worse ending.
Now that I have my own boat, there is no way I would dive with it unattended. Plus, I make sure the surface person knows how to operate the engine controls and radio - just in case. I also dive with a live boat quite a bit, which means I don't anchor, but leave the engine running and the surface person follows our bubbles at a discrete distance. Don't have to backtrack at all and the boat is always close in an emergency.