You might want to try electrolysis on it - you can make your own electrolysis kit quite easily.
If you'd like me to explain how I made my own electrolysis kit I'd be happy to post about it here.
Here are the before and after pictures of a badly encrusted silver ring that I found a few weeks back in the water at an ocean beach. It was not as heavily encrusted as yours though. I've also used electrolysis on a silver ring that was pretty discolored but not really encrusted. It had five stones set in it, and the electrolysis did not harm the stones at all. I have heard though, that if you have an item that has stones glued into it rather than set with prongs, that the electrolysis will melt the glue and the stones will fall out.
Anyways, here are some before and after pics of my encrusted ring...