I stayed at Cocolobo in Oct and thought it was lovely, even a bit too quiet for my taste (though it was low season and had only newly opened) so I'm sure you'll love it.
Land's End next door has loud live music and BBQ nights Weds and Sun. You can't hear them from Cocolobo but I was glad I hadn't chosen to stay at Land's End.
Just a quick report back - Cocolobo was exactly what we were looking for. Nothing too fancy, very relaxed, extremely quiet - you hear nothing but the ocean at night. The owners are having a small house built at the back of the property, so there's some hammering and construction noise during the day, but they seem to hold off until everyone is up in the morning. Rory and Claire, the owners/managers, are wonderful hosts. The hotel definitely gets a slightly older crowd - we were the youngest, in our early 30s.
Seagrape or Las Orquideas also looked like they would've been good options for quiet. We also heard that Land's End is trying to make itself into a bar destination, so were glad we skipped that. Half Moon Bay Cabins looked ok, but several nights the restaurant had really loud music on and the cabins are well back from the water. While we did snorkel off the beach at West Bay a few times, it was still far too resort-y for us. The best lunch option we found on the whole island has to be Mangiano, though - back off the beach on the main road, down the lane next to Thirsty Turtle. Excellent, huge, well-priced sandwiches instead of the overpriced fried junk all the beachfront restaurants offered. The snorkeling at West Bay was surprisingly great - we were really impressed given how easily accessible it is!
Unfortunately, because of the cruise ships, it may not be that way much longer. A very depressing number of cruise ship passengers were standing/sitting on or touching live coral. Some people didn't know better and didn't mind being (politely) asked not to, while others didn't give a crap and got very nasty. Really very sad and disturbing, and honestly just confirmed most of what I thought about cruises and the people that take them...