I've not done much with crab patterns on bones but the odd times when I did they worked great. Nothing special about the presentation but in each case there was eveidence of huge numbers of crabs in the area. One spot down in the yucatan was littered with millions of bleached fiddler crab shells. I aksed the guide, "what about a crab pattern?", he just shrugged, so I tied one on and the first fish nailed it. I've also had success with bigger crab flies (merkins etc) in deep water and the surf.
When sight fishing, look over your shoulder from time to time, you never know who's behind you