2015 350 mag mpi w/ bravo 3: Temperature fluctuates

Hi. I'm new to the forum. I've done lots of car work in the past and some work on my own boats. I bought my current boat new in 2016. The boat is a 2015 Stingray 225 LR with a 350 mag mpi and a Bravo 3. I mainly boat in brackish water. The boat currently has 275 hours on it. I'd like to say I flush religiously after every trip.....but I flush about 90% of the time. The other 10% of the time I'm lazy and don't flush until a couple days or a week later.....I know....not the best approach.

The boat typically ran around 150-160 degrees (as read by the gauge on the dash) until last season when the temp seemed to slowly creep up to about 170 or so if I was cruising at 40pmh or more. Typically I cruise around 28 mph.

When I winterize, I heat the engine up on a hose, drain the water (take out all the blue drain plugs and the block drains), reconnect everything, then run 5 gallons of RV antifreeze through the engine using a 5 gallon bucket connected to ear muffs. EXCEPT for this past winter. This past winter I just pulled the water lines off both sides of the block then took out 3 blue plastic plugs. Didn't realize there were 2 more in the seawater pump....duh....oh well....the pump appears to have survived.

I took the boat out for the first outing of the season last weekend. I noticed the temp rise up to 200 on the gauge. I immediately brought it back to idle, idled back to the dock, and took the boat home.

The thermostat and seawater pump impeller were original, so I replaced them. The impeller is a mercruiser part and the thermostat is a Sierra part. Went for a test ride today and the temperature was still going up to 200 then cooling back down to 170. Knowing that my cooling 'innards' were just replaced, I kept cruising around 27mph (~2500-2700 rpm) for about 10 minutes. The temperature continued to fluctuate between 170-200 at the gauge. This fluctuation is the same whether I'm at idle or cruising.

I have the mercury vessel view module / app on my phone. I connected to it and saw the temperature fluctuated between 150-160 on the dot (160 degree thermostat) as read by whatever sensor vesselview reads from. I touched the top of my catalytic converters and I could keep my palm on them for about 20 seconds then it was slightly uncomfortable. This is hotter then they were last year, but not scalding.

I started feeling hoses at the thermostat housing. 3 hoses were cool. The large hose that goes to the water circulator pump was warm / hot. I also felt the oil filter and it was hot. Couldn't keep my hand on it for more than a few seconds.

Now finally my question: what the heck is going on? The temperature did not fluctuate like that until this year. What is causing the temperature fluctuations? Why is the large cooling hose feeding the circulator (aka water pump) hot at the thermostat housing while the other 3 are cool? Why are my manifolds / catalytic converters hot to the touch when they weren't before?

One more comment: There is a cover on the top of my thermostat housing that is held on by 3 bolts. I took this cover off and cleaned the spring, the little poppet, and the area where the poppet seats. I didn't have a replacement gasket, so I put the cover back on with a thin film of rtv. Not sure what this poppet does or whether the fact that I didn't reinstall the kinda thick gasket will make the spring push down harder on the poppet.....but thought I would mention this just to be thorough.

Any help would be appreciated.
Feel free to ask me dumb questions because I'm not above making dumb mistakes

One more piece of information: This is a diagram of my thermostat housing:
https://www.hardin-marine.com/s-2656-thermostat-and-housing-standard-cooling.aspx

The hose connected to "1" in the diagram is the one getting warm/hot. Not sure if that hose is supposed to get hot or not. Any advice to help me understand the cooling flow of my engine would be appreciated. I tried to find a cooling flow diagram that showed me which hoses were cool water vs which ones were hot, but I had no luck for my particular engine / year.

Ok....so I think this comes down to a dashbaord gauge reading a little high and me being overly cautious / paranoid.
I took the thermostat housing off again to double check my work and get a better understanding of the flow through the housing.
I also read up as much as I could to understand the flow through all the stinkin hoses in this cooling system.
To add another point of comparison, I bought a mechanical temperature gauge and installed it right next to the temperature sender (going to the dashboard gauge) in the thermostat housing.
I also drilled a 1/8" hole in the thermostat (like I would in a closed car cooling system) as an air bleed. I realize I don't have to do that on a boat....but it made me feel better

I put everything back together and ran the boat on the earmuffs in my driveway. Just like when I was out on the water, the engine temp as read by VesselView slowly oscillated between 150 and160 just like one would expect with a 160 thermostat. The dash gauge went between 175-190. The newly installed manual gauge went between 155-165 or so. That gives me more confidance in the VesselView reading. Seems like my dash gauge is drifting off a bit.

I also monitored my manifolds. While idling at full temp, I could put my hand on them for about 2-4 seconds before I had to pull my hand back off. My water hoses going to the manifolds are nice a cool, so I think I have good flow.....and honestly.....I don't really remember how hot those suckers got last year.
Also, the big coolant hose going from the thermostat housing to the water circulation pump is supposed to be hot since it's flowing water that's regulated by the thermostat.

So.....in summary......I don't think I have a problem with the boat.....except for a slightly exaggerating gauge.
I'm going to take it for a long test ride tomorrow and see how it does.
I temporarily mounted the mechanical temp gauge in the engine compartment as a sanity check for the test ride.

If anyone knows how hot those manifolds with catalytic converters should be to the touch, please let me know.

I also made a powerpoint of the water flow through the cooling system and the thermostat housing.
I'll try to post that as a picture.....maybe it will be useful to someone.

[FONT=&quot]I made some assumptions about the flow through the water distribution assembly. If anyone knows for sure what's inside that thing, I'd love to know.[/FONT]
[FONT=&quot]Also, if I've stated anything that's incorrect in this picture, please let me know, and I'll update it.[/FONT]