Use the adjective churning to describe a liquid that's being powerfully moved around. A boat on a churning lake will be tossed around on its surface.
A churning sea is the result of a violent storm that blows against the water and produces large waves. You could even describe your churning stomach when you're incredibly nervous about giving a speech or making a presentation. In either case, there's an intense kind of agitation going on. The root of churning is the Old English cyrin, related to cyrnel, or "kernel," which describes the way churned cream looks grainy or gritty.