District Judge James Dannenberg also ordered Watros, 37, to undergo an alcohol assessment and to submit to 14 hours of counseling.
"I'm sorry," Watros said after entering her plea.
Under a plea agreement with prosecutors, a citation for crossing the white line was waived.
"This is no fun, but you know how much worse it could have been. Get on with your life, and I wish you luck," Dannenberg told Watros.
Michelle Rodriguez, who co-stars with Watros as police officer Ana Lucia on the castaway drama, was also charged with drunken driving. The two were pulled over in separate cars within 15 minutes of each other Dec. 1.
Their cars had been spotted weaving on a road in Kailua, on the island of Oahu, where "Lost" is filmed. Both failed field sobriety tests and were released on $500 bail, police said.
Rodriguez, 27, who was scheduled to stand trial March 30 after pleading not guilty last month, was also cited for speeding in Hawaii three times last year.
In 2004, Rodriguez pleaded no contest in Los Angeles to three traffic violations, including drunken driving. She completed a three-month alcohol program and is serving a three-year probation term.