Go to
www.mercurypartsexpress.com The site will give you exploded views of your engine etc along with part nos.

Youll need your motors serial no.A postcode in the US I use 60438 & then pick any dealer.The impeller/waterpump is located above the lower unit on the driveshaft,in the center section .

Here in Australia it is cheaper to buy a water pump repair kit which includes gaskets, base plate,impeller& connection tube A$55 as opposed to

A$69 for the impeller alone

But you should get a manual it will guide you thru the maintenance reqd.