Full strength vinegar and a Mr. Clean Magic Eraser worked great on my boat that had some stubborn spots up under the rub rail. Full strength vinegar and a rag by itself wouldn't take them off, the Magic Eraser did the trick.
Also, just an idea, try half CLR and half vinegar. I did this to my shower heads the other day and it got them looking completely new. CLR wouldn't do this by itself in the past. I'm not sure if mixing them is a good idea or not, but it didn't explode or smoke or anything....