Tenax, I gather you dropped the whole utility knife in the pool and not just the blade. In either case I'd take the vacuum head off the pole, tape the hose to the pole, then approach the knife from the handle end and the suction should suck it right up the hose into the skimmer. It should fit inside a typical 1.25" diameter hose. Assumes your vacuum hooks to a skimmer. Also, a good source of strong magnets are old hard drives.
Al