How can I vent moisture from a small pump house?
Unless I am missing something, this seems a perfect application for an air-to-air heat exchanger.  Fully insulate the building, seal it up, and install the heat exchanger in a convenient location.  A blower through it could be controlled by a humidistat.

If winters are very cold, some supplemental heat might be needed inside the building unless there is quite a large mass of water.

A reasonably effective plate heat exchanger could be built by anyone with basic sheet metal experience.