The national park rangers office is located on Koh Wua Ta Lub just behind a pretty white sandy beach with turquoise sea and a few langur monkeys that can sometimes be seen at the right end of the beach where a trail to Tham Bua Bok cave and its flowering lotus-shaped stalagmites and stalactites starts. Koh Hin Dub has the longest and most spectacular beach in the national park.
I recommend this tour to visit Ang Thong Marine park (I tested it myself).
To go visit Ang Thong National Marine Park, you can also book one of the tours above.