Water is a type of terrain in Erfworld.
Known Types of Water Hexes
A river hex might count as land and water, allowing entry by both land and Water-capable Units. It would allow passage of water capable units only between two particular sides of the hex, while denying land-based movement across the river, unless a ford, bridge, or ferry was present.
Rivers flow in a certain directionIPTSF Text 38. Traveling up- or downstream could possibly affect move cost. It's unknown if the river's flow can take units that won't move (like incapacitated units) and items across hexes, or if they get stuck at the hex boundary.