Was this for a 3YM20- or some other engine? If a YM... First, there is a raw water filter- probably on the starboard aft side of the engine, on the wall. Close the thruhull intake! Unscrew the cap and lift the basket out. You'd be surprised what can get trapped in there. BE SURE to fill the filter after you put the basket back in!!! This is to prime the water system. Be sure the rubber gasket is in the cap!!! It will take maybe 15 seconds before the water comes out the exhaust thruhull.
At the front, lower, port-side, of the engine there is a hose that supplies water to the engine-mounted impeller-type water pump. On the hose is a 1" or so protrusion that you can take apart to drain the hose back to that point. You might have to destroy the band clamp to get it off, but get a regular ( 1 inch?)hose clamp before you even start taking it apart. Bits of my impeller were there, and maybe the blade, as I found it under the engine later on.
Looking for pieces of rubber in the heat exchanger, I took off the front cover and alternated blowing/sucking into the tubes with a small shop vac. The rubber gasket/ring is probably available at a hardware store plumbing department. For my 3YM20, heat exchanger "O" ring, I show part # 128-990-44100, size D6/5,6/3.53 (I will recheck my spare for that size.).
Wash and rinse the air filter. They are expensive, IIRC.
I keep rubber spares in plastic jars to prevent them being crushed. A used spare is better than no spare, so keep the one you take out. GET A LOG BOOK and write every part number you buy. For the engine bolts, I write the wrench size next to the bolt with a Sharpie.