Well I've been looking around in the tutorial world to see what's new, and here's what I've found so far, secret or not:
-New piston tutorial where you start off
-Self building bridge in front of the castle.
-Chest filled with diamond equipment and armor in an underwater tunnel near the pier.
-Nether portal inside the Minecraft logo
-all the old secrets (lighthouse, secret village, secret room under the church, etc.)
I'm confident there is more stuff to find. Anyone else find something by looking around?
I found it to. Go out on the pier under the castle. Swim to the left until the stone turns into dirt. Look back toward the horizon and swim out five or six blocks or so. Look down in the water and you will see a faint light. Swim all the way to the bottom of the lighted tunnel and then swim across. The air you have will allow you to just nearly make it. Search YouTube for video but it should be easy with what I posted.