My dealer started a new boat in the showroom and man did it sound sweet! I personally never start it out of the water. I do exactly what someone else recommends I leave it on the trailer and back it off in reverse. I don't like the push off the trailer due to anything can happen and if it's still on the trailer you really don't have any big problems. (leave the plug out and etc..)
I'm going to bet it's not the impellar, I think it's something else.
Let us know what you find.