I'm thinking of those resin terrain pieces that are babbling creeks. I'd have a hard time saying that a two inch wide idyllic creek would hold up a walking warmachine. Open water, lakes and rivers all would deffinatly halve movement. Really it just needs to be agreed upon beforehand, like it says to do in the book right after the description of terrain types.
-PFC joe
Then again, a gently babbling brook really wouldn't hinder a person either, unless they were trying not to get wet.