I loved the Full Moon Party, but once was enough. Afterwards I'd prefer to stay somewhere a bit more quiet and Haad Rin Beaches are all about parties. So if you don't mind staying up late, constant noise, music, drinking etc - you're going to love this area!
To be honest, any spot in Ko Phangan you choose, is probably going to be great for snorkeling. The only difference is an access - if it's hard to get there it's not going to be crowded.