It is possible now to travel by ferries in high season (Nov-May) from Phuket and island hop your way down the coast all the way to Indonesia.
This can now be done without ever touching the mainland,
Phuket (Thailand) to Penang (Malaysia), islands en route:
The Thai portion can be done in a day.
Ferries cross from Satun in southern Thailand to the Malaysian island of Langkawi, while over in Narathiwat Province, a vehicular ferry shuttles between Tak Bai and Pengkalan Kubur, near Kota Bharu in Malaysia's Kelantan state.
There are also occasional cruises from Malaysia and Singapore to Phuket and Bangkok, the main operator being Star Cruises, but no scheduled services.