More than 100 tourists have been stranded on Koh Tao in Surat Thani as torrential rains batter the resort island and other parts of the South.
Samui residents were afraid to leave their homes as there was no power supply for a second day.
Bangkok Airways cancelled flights between the capital and Samui because the island airport's runway was flooded.
Flights between Phuket and Samui were also cancelled, affecting about 100 passengers, mostly foreigners.
All South-bound trains were forced to stop at Tha Chana railway station as floods have made tracks impassable.
Mobile phone connections have failed in many areas. Residents could not withdraw cash in some areas as many ATMs failed to function.
Floods in Phatthalung have claimed four lives since Monday. Three died in Bang Kaew district and one in Khuan Khanun district. Most were washed away by flash floods that have affected over 30,000 families, or 102,000 people, and damaged 70,000 rai of farmland in 11 districts.