I have just returned from spending 3 days on Koh Talu, a tiny island off the coast of southern Prachuab Kirikhan, about 200km south of Hua Hin. It took just over 2½ hours by car to reach the ferry port, which was actually a beach near Bang Sapan, & then we had to wade out to the small boat -- don't come wearing designer clothes! There are 2 resorts on the island, the smaller & cheaper Mook Bay (very popular with day trippers) & the larger, more expensive but much more attractive Big Bay resort. We stayed at the later & were glad we did. Not only is the beach bigger & cleaner, but folk using the Mook Bay hotel usually came to our place for lunch & for excursions -- which must say something! I did enjoy our time there but when we went out snorkeling near the truly spectacular cliffs (the island's name means hollowed-out cliffs), I slipped under the boat & the barnacles rather changed the appearance of my legs! I didn't notice a thing until we returned to the resort, when the lady at the beachside bar almost screamed & pointed to my rather bloody legs. However, she & the 'guest relations' guy (nice chap named Lek, look him up if you go there) treated my leg & cleaned it up, so a sort of happy ending. The resort is very nice & as we were the only guests (the rainy season's coming up & it was midweek), we were spoiled by the staff. However, take a mosquito coil with you, as the rooms don't have mosquito nets & the electricity isn't 24-hour on such a remote island. However, it's worth a trip, though a day trip might not be so worthwhile -- you only get to spend about 4 hours on the island!
(If you'd like to visit there, I recommend the agent we used. If you'd like to know more, let me know.)