I have a 87' Rinker V170 and i think the impeller is going bad, how hard are they to change and is there anything i should be careful with while i am changing it.
Being as it's an 87, and a 120, it has to be an Alpha I outdrive.
Here is a link to an online service manual for your outdrive. http://www-alt.mercurymarine.com/mnetdata/SERVICE/CRUISER/Servmanl/6/6covr3.pdf
Just scroll down to the outline page and click on the section letter you want to view.
Section 3B shows the raw water pump.