The raw water pump is a KEY component of the boat's functionality. To have missed its imminent demise would be egregious in a pre-sale inspection IMHO, but only IF it was making that noise when they inspected it (again, the timing would certainly be coincidental if it looked and sounded normal at that time). If I were buying a used boat, I would want a close look at the raw water pump to look for leaking (which usually indicates a bad seal). Certainly, if I heard a grinding sound from ANYWHERE in the engine compartment, I'd consider that a real problem and would want it tracked down.
I guess a key question is why they told you that the impeller needed replacing. Did they say that because they just figured it was time, as part of routine maintenance ("change the oil, change the filter, change the impeller")? Or did they note that the boat was overheating, or (worse) did they hear a metal-on-metal grinding coming from the pump and figure (erroneously) that it was just the impeller? That's a bad assumption and suggests that, at the least, they are unfamiliar with inboard boats. If it was just silently overheating, a bad impeller would be a reasonable assumption, although for $350, it seems MORE reasonable that they would take the 10 minutes and pop the pump cover to actually look.
FWIW, a total impeller failure is absolutely silent and is suspected by the boat overheating, and actually diagnosed by removing it and directly observing it's loss of vanes.
'04 MariStar 230VRS/MCX