Several years ago my brand new, never been used 45# Delta was hooked solidly on a some object in about 15 feet of water, I was not too far from an abandoned oil platform so probably part of it. I tried all maeouvers I could imagine with the boat, pulling from all directions, tighten, slack, etc it wouldn't budge. As I was about to give up and put hacksaw to chain I remebered something former TF member Charles told me years before. I had a short chain and made a loop about 1' diameter around the anchor rode. I tied it to a rope to the loop, pulled the anchor chain tight and vertical above the anchor. I lowered the loop down the chain it slipped down and around the anchor shank, I then loosened the anchor chain quite a bit and fastened the rope to a cleat and drove forward, as the boat moved forward the loop of chain and it's rope pulled the anchor free. It worked fine and saved my brand new anchor. I have never had to use the method again so no further experience.
That was on a January 13, with ice on the handrails that morning, the reason I didn't go swimming for it.
Thanks again Charles,