This island group is not to be missed and you should plan a tour to them during your stay in Koh Samui. There are several options: both from Bophut and Nathon large tour boats leave every day (weather permitting). They need to two hours to get to Anthong and then hop from one island to the other. The boats always stop at Koh Mae Koh which has a lovely beach, called Mae Koh, which is surrounded by toweing walls of craggy cliffs,  and an interesting turquoise-coloured saltwater lake (Thale Nai) in the middle of the island (warning: the climb to the lake is not easy, but rewarding). 

The main island of the group is Koh Wua Talap (Island of the Sleeping Cow), where the park office is located (and also a small bungalow-style guesthouse for people who would like to stay overnight). The main attraction, beside the beach, is a large rocky hill which you can climb to admire the stunning panorama below. 

And there are countless other little islands, a number of them with good snorkelling opportunities (e.g. Sam Sao). In Koh Wao and Koh Tungku, the two most northern groups of islands in the Marine National Park, large schools of fish can be seen (up to 1000). The island also offer interesting caves that are safe for diving.

If you do not like the large tour boat, there are also speed boats that leave from Bophut or Nathon (and can reach Anthong in one hour instead of two).  You can also arrange private speed boat excursions through some of the hotels.

The costs for a tour with the public "slow" tour boat (the ticket should be bought in advance from one of the small travel agent offices in Nathon) start at 250 Baht per  person; speed boat trips start at 800 Baht per person.