There is a passenger ferry that crosses from Samana to Sabana de la Mar. I believe it goes three times a day. I took the 9 am ferry from Samana in May and it cost RD$25 and took about an hour and a half. There are seats inside or outside on the deck if you prefer. Note that when you get to Sabana de la Mar, the harbor is too shallow for the boat, so it anchors about 200 yards from the pier. You then transfer onto a smaller rowboat (actually pulled by locals who are up to their necks in water) to the pier. This will cost you an extra RD$10. There is very little to see or do in Sabana de la Mar but there are frequent guaguas to Hato Mayor and San Pedro de Marcoris.