Amazing hike but it is very challenging. I've done it twice once with rain and once without. With allot of rain it's even harder...and dangerous. Bring lots of water and proper hiking shoes you'll be allot happier. The falls and the views of the coast make it all worth it.
I was there when I was stationed at MCAS Kaneohe Bay in 1966. It was the brightest , and most colorful ocean habit that I had ever seen.
I snorkeled here almost every weekend and I remember the bay water was azul in color and incredibly clear . The bay was easily accessible for anyone who could walk by a long steep stair way of stone steps with pipe guard rails .
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!