"How does the boat unhook the grapple from the wreck? "
Very good question - That's where the lift bag comes in - and a reel - you attach the reel, and the lift bag to the anchor - remove the anchor, inflate the lift bag, and it will go to the surface - and you are "attached" to the lift bag and the anchor because you left the bail open on the reel (right??)
This way you don't have to lug an anchor up - the lift bag does it for you, and you can come up safely as you reel in as you go up, do your safety stops - avoids you being separated from the anchor so the boat just comes and pickes you up - although in the interest of full disclosure, sometimes the reel and lift bag go up together w/out me.
On a more serious note, none of this stuff is rocket science, but it does take practice - especially finding and hooking the wrecks from the surface - we have been doing it since 1985 from our own boat, and some days (often because of current) it just does not happen.
And you are far better than I if you can throw the anchor/grapnel from the bow - or use an automated windlass - and hook something - I stand in the stern, let out the anchor when we are over the wreck, and "feel" it catch - doing it w/out the hand feel I think is a way to be frustrated quickly. I give it 3 tries, then send down a diver w/ the grapnel.