There are a lot of ways you can use a reel to help find the anchor -- if you aren't diving a wreck, you can run the line down over the top of the wall or reef, to mark your starting point. Or if you are diving a spot with little depth contour, just stretching the line out 50 or 100 feet can give you a much bigger target to hit. (In that case, though, I WOULD tie to the anchor line, or you run a real risk of losing your reel if you still don't make it back.)
If you are talking about diving off a private boat with a boat tender who can't run the thing, think seriously about whether you want to do this. If, for ANY reason -- current, equipment malfunction, navigational error) you come up where you can't swim back to the boat, you AND the person on the boat will be in a world of hurt.