Go along with prevailing opinion or thought, as in Irene doesn't have a mind of her own; she just swims with the tide. In the late 1600s this idiom was also put as swim down the stream, a usage not much heard today. The present form was first recorded in 1712. For the antonym, see swim against the current.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.