Abstract: A device for aerating or mixing a body of water has a mechanical displacer driven by connection to a hydraulic motor such that a flow of liquid supplied to the inlet moves the mechanical displacer so as to displace water of the body of water. The device may be supported by floats or mounted on a fixed structure. The device is preferably actuated b a flow of water delivered via tubing from a pump located remotely from the body of water, thereby avoiding, the wet-environment shock hazard of electrically driven aerators and mixers.