I was fortunate enough to be told about this hidden beach and I'm really hesitant to share the location out of fear of people overcrowding and ruining it.  At the same time it's an amazing place and people should have the opportunity to appreciate it.  It's called Platja Coll Baix and is located on the northern side of Mallorca near Alcúdia.  It's only accessible by hiking 2-3km or by boat.  When I arrived there were only 3 other people on the beach and a single boat anchored in the cove, while I was the only person actually in the water.  The visibilty was amazing and the water temp was perfect.  I highly recommend bringing fins because even though the water may look calm, you never know what kind of currents are coming through these places.  I actually lost a friend down in Mexico a few weeks ago after he was caught in a riptide.  He was a strong swimmer, but didn't have fins on.  Anyway, fins (I was using Churchills) and a quality mask (I was using a Riffe) will make the experience much more enjoyable.  After hiking and climbing over some rocks to get to the beach, jumping in the water was a refreshing reward.  Up to this point this beach is definitely one of my favorite spots I've been to.