Built by Liquid Robotics, the PacX Wave Gliders are taking part in a project to gather data about the composition and quality of seawater.
The first leg of their transoceanic swim from San Francisco to Hawaii took four months and they are expected to cover 9,000 nautical miles by their journey's end.
They were launched in San Francisco harbor Nov. 17, 2011.
The robots consist of a floating upper part, shaped like a surfboard, connected by a cable to a lower assemble that has fins and a keel.
They have no fuel or engine, instead converting energy from the movement of ocean waves into forward motion.
Solar panels installed on the upper surface of the power sensors that take readings every 10 minutes of the surrounding seawater.
"I have no doubt new ocean discoveries, insights, and applications will emerge from the PacX data set," Edward Lu, chief of innovative applications at Liquid Robotics, told the BBC.
After a short stopover in Hawaii, the robots will split up, with two going on to Japan and two to Australia, crossing the equator.