I think the reason the pirates didn't want her to drop the coin, was that if she did drop it, then it would drop the bottom of the bay, were there are plenty of rocks and places were it can get lost and hidden. Even today, it is very hard for people trying to find shipwrecks to find exact things. The other two reasons I can think of is that the Pirates were lazy and didn't want to waist the time, or it was just a script error. There was one other script error that you can see in the deleted scenes of the DVD.
Also, the possibility that Bootstrap was able to escape is possible, and might show up in a sequal. It would be a good way to keep William Turner (Junior) in the story. But, it all depends on were he was dropped. After watching the movie a dozen times, I can pretty much understand the limitations of this curse. If he was dropped in a deep enough area, he would be crushed. Being alive and being so low in the dark by yourself probably seemed like hell. If you got away, it would probably make you want to be more of a christian.