It's only a theory thrown about by some. One of many. Thor Heyerdahl sailed a raft across the Pacific from Polynesia to South America back in the '60s demonstrating it could have happened. There are some controversial archaeological sites which have been dated before the Clovis barrier of 11,000 years ago. Clovis has long been the oldest evidence of humans in North America. But there are a few sites like Monte Verde in South America, which some people believe to be older than 11,000 years. Some have theorized that Monte Verde suggests a Pacific migration, while others believe it suggests a coastal migration down from Alaska before a migration into North America. Pedra Furada in Brazil has been dated to 50,000 years ago by some. Of course this is a very controversial date. Quebrada Jaguay, and Quebrada Tacahuay on the south Peruvian coast are at least as old as Clovis and show a degree of maritime knowledge and adaptation. This suggests to some that a Pacific migration could have been possible, while others interpret these sites as evidence of a coastal migration.
You can read about Monte Verde here:
http://www.unl.edu/rhames/monte_verde/MonteVerde.htm