Once you've seen an impeller, you will understand why all of these people are saying to change it out.
Impellers are made out of a VERY stiff rubber. They are basically a paddle wheel that spins on a shaft VERY FAST. Their job is to push water, and lots of it. Stuffing it into your engine's main circulation pump so it can then be redistributed through the engine block, the exhaust manifolds, and even your hot water shower.
In this picture http://www.tigeowners.com/tech/images/impeller7_t.jpg
the impeller has failed. All of the veins have sheared completely from the impeller hub. The impeller is the mass of black rubber by the blue arrow in this picture. It should normally look like a ninja throwing star. Once installed, the veins should be bent over like several squeegees, or the thing they threw in that that old movie. Krull?
Now your little paddle wheel is 2 years old. I would guess that after sitting in it's installed (bent) position for two years, the veins of the impeller are basically like two week old road kill. STIFF! Imagine how stiff your rubber windshield wipers are after one summer!
The pulley that drives the impeller is smaller than the main crankshaft pulley. This causes the impeller to spin at an RPM faster than your engine.
So this brittle rubber paddle wheel spinning 5,000 to 8000 RPM inside of a cramped housing pushing water to your precious little engine. I guarantee, it will fail before the season is out. When it does fail, you will probably have a boat full of friends ready for a great day on the water. Just like the Soup Nazi. No Boating For You!
$60, $100, hell $200 is cheap insurance that you won't have a ruined day, and a blown head gasket next time you hit the water.
It's really not a difficult job to change it yourself. The first time I did this, I found a sheared bolt on the housing. Needless to say I was a little cranky about the last maintenance service I paid for. NOTE - NEVER EVER EVER, run your engine without a water supply for the impeller. The impeller itself will overheat itself from the friction in the housing and fail with a loud screeching sound.
Don't ask me how I know that.