Will learns to play the piano from a book Grace buys him, however at the end of Season 3 Ep 5 it is shown that Will already knows how to play.
When Will is dressed in scuba gear, he gives a thumbs up to mean 'all is well' however when scuba diving, this gesture means that something is wrong and you want to return to the surface. The gesture for 'all is well' is making a 'o' shape with your thumb and forefinger.