One way is to disconnect the raw water intake hose from the through-hull valve and immerse it in a 5 gal bucket, placed alongside the engine and filled with freash water.
If you plan on running the engine more than a few seconds, stand by with a garden hose - filling the bucket as needed.
You could also close the raw water seacock, remove the cover of your raw water strainer, then insert one end of a 6 foot + - rubber hose into the top of the strainer, then the other into the bucket.
True Blue . . .
sold the Nauticat