1975 Mercruiser 302 Ford - 225S

I have had this boat, an 18' Glastron since new. It has a fresh water cooling system which I religiously drain every fall for the winter. Then in the Spring when putting it into the water it always heats-up so I would shut the engine off for a few minutes, then start it up again to see if the temperature guage would come back to normal. If it did it would be fine for the season, if not I would repeat the process again until the heat guage would stay at normal. This was a yearly process.
Over the years I've replaced the lower leg pick-up pump impellor, waterpump, thermostat, checked and cleaned the exhaust manifolds etc., and every year at start-up it always does the same thing. At Spring start-ups with muffs on I've tried warming it up for 4/5 minutes, pulling out the brass manifold plugs, starting the engine until water comes out of the the manifold plug holes, putting the plugs back in and wait for maybe 45 minutes - this sometimes worked and other times did not.
After 43 years and this mystery every year I'd sure like to know what the issue is.
Are there any fine gentleman out there that may have some solutions ?
Regards,
Jaytan

 

What engine and model does it have?

 

How about a photo as the Merc kit on a 225 included the manifolds in the freshwater so I would like to see what you are removing and get raw water flow?

 

Jaytan - Am attempting to send pictures of my water manifold as requested, two (2) of them - looks like I have to send them separately, maybe I'm wrong but will try anyway.
Regards,
Jaytan

 

Jaytan - this is the second of two (2) pictures I'm sending and hope you receive them in answer to your request for pictures of the manifold assembly.
Please don't tell me I have been taking out the wrong manifold water plugs for 43 years. The ones I have been taking out are the 3/8" 24 TPI Brass plugs and not the larger square socket Cast Iron plugs.
Regards,
Jaytan