Replace them. Here's the deal, if you don't replace them you save maybe $150 or so and a few hours of your time. However, if you're out on the ocean, Sound, bay, or whatever body of water you're on and they go up on you and you're 60 miles from home then you're in a world of sh!t. Towing, possibly being stuck with weather on it's way, plus the fact that you have to replace them, fish out any broken off pieces in the cooling system (if it's even possible), will make you kick yourself for not doing it in the comfort of your own home or marina while sucking down a couple of beers listing to a ball game as you're doing it.

After 35 years of boating I've learned that if you temp fate you'll get stung. Boats do nothing but break down and need tons of maintenance during the best times, why push your luck?

Very true! I am the kind of guy that likes to carry spares for just those reasons.

Impellers are a maintnance item. Dont even think about it CHANGE THEM.
I also keep a spare under my seat. If not for me maybe someone else.