If it's a new engine and you plan to run water only, drain the block after each use while the engine is hot. The heat will dry out the water fast enough so to not rust up the internal water jackets. A used engine is likely coated already and won't see rust like a well cleaned block will. I've broke-in enough new engines to know and have design my test stand with water drains from each side of the block. Antifreeze is fine, but is a slimy mess if you need to work on it.
