For the dolphin. They aren't really unlockables, but they do help.
Tail Walk - touch on fin
Sing - Rub on fin
Spin Jump - tap on mouth
Backflip - grab on mouth
Handshake - grab on fin
Front Flip - rub on mouth
This all the songs we have unlocked so far.
Pokarekare Ana - save the bottlenose dolphin
Prayer - it's at the beginning screen
The Water is Wide - View credits (gaze at ocean around 6 pm (clock in cabin))
Shenandoah - go in the mermaid's cave
Benedictus - Abyss
Amazing Grace - Go to the ruins
View Staff Credits
When it's sunset in-game (approx. after 6pm on the clock in the boat's cabin), sit on the deck chair, and look out over the horizon as usual to view a special cut-scene with Staff Credits.