I'm still trying to get more information about Stone Island, but here is what I've discovered... you can take a water taxi over to Stone Island for about $1/pp round trip. Stone Island has a couple of restaurants (supposedly great garlic shrimp), and you can go on the banana boats, snorkel, horseback ride and boogie board for about $3/pp for each activity. You can go to www.mazinfo.com
for more info.
Don't know anything about El Cid, but I'll be interested in what you find out.