To get to Puerto Galera, you have a couple of options. One, is a RORO. That means Roll On, Roll Off. You can drive your car right onto the boat and drive it off on the other side. This is a good option if you are exploring more of Mindoro or headed to Abra de Ilog. If you are going a resort in Puerto Galera, a better option is to book a private banka (boat) through the resort you are staying at. Sometimes if this happens, the boat will be able to drop you right on the beach of the hotel.
Diving options are available at all resorts or really close. Snorkeling is done from a boat in some great spots. The resorts can tell you what they have activity wise to offer.