There was a thread here recently about someone loosing an impeller vane or

something like that after running a motor very briefly out of the water.

That would likely have been my problem. This was with a small outboard (4hp)
and I ran the engine very briefly out of water and it trashed the water
impeller. I got the damaged one back and I can see why it was destroyed. The
impeller has rubber vanes which make direct contact with a hard plastic
housing. Use in water works just fine as the water acts a lubricant and
removes any heat real fast. But out of water, it would be like taking a peice
of rubber and rubbing it on a peice of hard plastic. It would heat up very
fast and leave rubber on the hard plastic in no time at all. Once a little
rubber gets put down on the hard plastic, the friction goes up, more heating,
more damage..

Dont know whats in your configuration but my advice would be to not run the
engine at all out of water. Its not about overheating of the engine, its all
about the water cooling pump getting destroyed and costing you a big hassle.