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.
You have to book a tour with Oxalis which is a monopoly tour provider in Phong Nha. You cannot visit the cave otherwise. This trip is 2 days trek, they provide most of the equipment and food which is amazingly delicious. It's one of the best experiences in Vietnam!
We had a fantastic 1st visit to Mammoth Cave! Ranger Aaron was a great interpretive ranger, mixing the geology of the cave with a framework of the cave’s historical significance and the guides that had been a part of that history. So glad we made our reservations online a few months ago.
A great experience and incredible opportunity to visit and see inside a glacier.
The tour guides were really helpful and knowledgable and the drivers (on the shuttle to base camp and on the glacier) really skilled.
Don’t forget waterproofs and waterproof shoes…although can borrow when you arrive at base camp. Also a small torch helps to throw light in the glacier to help with photos.