The snorkeling at Cua Dai Beach is known to have good sights. To reach the snorkeling in Vietnam by air, you will most likely need to fly into Tan Son Nhat International Airport (SGN) located north of Ho Chi Minh City. Alternately, you may be able to get a flight into Noi Bai Airport in Hanoi. Once in Vietnam, there are several domestic airlines, such as Vietnam Airlines and Jetstar, that have routes to the remote cities closer to the dive destinations. These domestic flights can cut a day or more off travel times by bus or train. With that said, the railways in Vietnam do happen to serve the coastlines reasonably well and might be worth checking into for your itinerary. Keep in mind that the Tourist Visa for Vietnam must be issued before arrive.
Overall, Cua Dai Beach is the 4th most popular snorkel dive spot of all 7 snorkeling dives in Vietnam.