Southwest of Phnom Penh off the southwestern coast, the snorkeling in the remote Tho Chu achipelago is known to be excellent. The Tho Chu archipelago consists of 8 islands in the Gulf of Thailand and is roughly 220km from the Vietnam coastline. Some of the best Tho Chu snorkeling is at Tu Islet where there is plenty of healthy hard coral reef formations that provide habitat to diverse marine life.
To reach the snorkeling at Tho Chu from the mainland, you will need to take the regular boat service that stops over in Phu Quoc Island for the night along the way and the total travel time is 16 hours. 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, Tho Chu is the 7th most popular snorkel dive spot of all 7 snorkeling dives in Vietnam.