If your lucky, the stock Sherwood pump has been replaced with an oberdordfer, which takes 10 minutes or less. If you have the Sherwood you pull two bolts and remove the pump to access the impeller Say 30 minutes You Tube videos should be there for both pumps

A photo of the front of your engine (towards the top) should help us, but it probably says right on it.  You're looking for a cylindrical housing attached directly to the engine (not a fan belt) that is approximately 3-4 " in diameter and 2 inches thick, like a can of tuna mounted horizontally with the lid facing the front of the boat.

Pro tips: Remember to close the raw water through-hull before you crack into your raw water system, or you'll be diving for the seacock in a hurry after you open things up!  I don't pull the pump off the engine to replace my "late" Sherwood impeller.  I just remove the three 7/16" bolts (backwards! -- who designed that?) and fold the housing down with the hoses attached. The impeller stays on the shaft and I just replace it. Remember the order of the seals when you take it apart - paper, metal, so it goes back together the same way.  I use a bit of KY jelly on the fins for lubricant, so it doesn't run dry at the first startup (overly cautious). It's water soluble and will not damage the rubber impeller blades. Some folks use dish soap for the same purpose.  Use a small box wrench for the 7/16" bolts so you do not overtighten.  

BTW: Here's the page on Catalina Direct for all the engine maintenance stuff, so you can order your spares to have on-board  -- ​https://www.catalinadirect.com/shop-by-boat/catalina-36/engine/maintenance/   You should always have at least one extra impeller, engine V-belt, fuel, and engine oil filter when you go out. They may not be the absolute best prices available, but at least they're guaranteed to fit your application, if you are risk-averse as I am.