Getting to Koh Sdach is best done by road, heading to the small fishing village of Poi Yopon on the mainland to then be picked up by the resort for a 10-15 minute boat ride across to the island. Poi Yopon is also the pick-up point for guests heading to neighbouring Koh Totang Island. You can also get a local boat that goes to the other end of the island where the village is. These run all day, when they have business. There isn't a schedule.
A taxi to/from Phnom Penh costs $90 one-way (4-5 hours), Sihanoukville $80 (2-3 hours), Koh Kong border $60 (2-3 hours).
Alternatively take a local minibus and get out at Cafe Sok Srei, six kilometres after the town of Andong Tuek coming from Phnom Penh. From here catch one of the local minibuses ($7.50 per person) to Poi Yopon, which takes around an hour to reach along the newly built road. You will need to have left Koh Kong, Phnom Penh or Sihanoukville on the first bus to make sure you have enough time to catch the minibus as these do not run in the afternoon.
The local fishing boat leaves Sihanoukville every other day at around 12:30, taking six hours to get to Koh Sdach, It departs either from the Royal Pier or the Thmor Sor pier, costing around $10 per person. It's not comfortable so road is the preferred option. Alternatively you could splash out on a private charter; Belinda Beach Lovely Resort suggests Captain Ronny's sailboat with Sail Cambodia -- $300 for up to 10 guests.