I guess we are all saying the same thing, I think, just different ways. The SS baffle/plate is between the outer exhaust cover and the exhaust manifold itself:
#48
http://www.marineengine.com/parts/johns … +Crankcase
So, what Dan is saying is that exhaust back pressure might stop cooling water from flowing if this baffle/plate was rusted through. I agree except that I would think you would be getting water into the powerhead if this baffle was rusted through, BUT anything IS possible.
The plastic guide is not used on all models/years:
#114
http://www.marineengine.com/parts/johns … sing-35+Hp
This plastic piece is simply a guide to help correctly install/guide the water tube into/through the upper grommet during installation. True, it will melt during an overheat, but it won’t block/clog the cooling system. The upper grommet may get crushed by salt/melt, but it sounds like this engine is a fresh water engine that has not been severely overheated.
My money is on a messed up/melted water pump, misinstalled/bent/kinked water tube. I’m thinking the gearcase to SS impeller plate mating surface is OK because you state this is a fresh water engine.
The later model engines use a plastic washer as a guide instead of the plastic guide:
#72
http://www.marineengine.com/parts/johns … n=Gearcase
The picture is misleading because the shoulder on the water tube is actually on top of the water tube and secures the plastic washer/guide in position. Is the impeller housing spacer/orings in place?
#64
This style water pump does not have a conventional seal in the plastic housing and should have the little vent hole you are talking about. You mention installing a new impeller, did you use OEM or aftermarket? You may want to invest in the complete BRP water pump kit with the plastic housing to ensure none of the things Dan mentioned about the water pump is an issue.
Finally, please do not attempt to replace the front half of the crankcase to take care of the messed up threads. These parts are a matched set, and can not be mixed/matched with pieces from other engines.

That SS baffle plate is not going to rust through. I have never seen one burn through either. It is actually pretty thick, but it warps and pulls away from the gaskets. You will know when you see a warped one. This causes the exhaust pulses to enter the water flow and it overcomes the pumping action of the pump. It is not enough that it burns up the pump, but it keeps the powerhead from getting enough water to cool properly. I do agree something else happens first! Someone will go in and put in an impeller, but that will not cure the whole problem. The water pump is fairly complicated in it construction and assembly. As Don said the shift shaft running thru the middle of it doesn’t help. These little motors have been prone to overheat since the beginning. I don’t know of any motor OMC engine ever made that has a fiberous washer between the ignition coil and the head. It’s because this motor has an overheat history and will get hot enough to melt the coils off the engine! That’s hot. This motor is also a Siamese cylinder motor. It has no full water passage between the cylinder. If gotten hot enough the cylinder sleeves will separate from the aluminum casting. If that happens you need to go looking for another block. That cylinder will not cool properly from that point on.

Dan in TN

Yeah, hate the silly design, but the plastic keepers aren’t gonna fail.