I just replace mine. I ran the engine for a few minutes with the seac0ck closed. It was a great excuse to have a mechanic look over the whole engine, and to give me a diesel engine lesson. The impeller looked like new. I replaced it anyway and kept the old one for a spare. Note that you need to get a spare gasket (mine are paper and are destroyed at time of removal) or you won't be able to use that old, usable impeller.
It was also a good time to swap out the antifreeze, which turnd out to be a bit watery. This may be the boat's first winter north of Georgia. Also it had "green" antifreeze, but Perkins engines like "blue" apparently.