There is nothing wrong with 160 water temp and 180 oil temp. Your water pressure does seem low still though.
have you checked your outdrive to make sure nothing is clogged in there?

I haven't removed nor disassembled the drive. Those who have, what size orifice are we talking about? 1"? Wide open, and variable in size? I've never seen inside of an upper or lower gear case, unfortunately (or fortunately!).

I've got DWP with Hydromotive nose cones. All holes on upper pickup are open (no pipe plugs installed). I thought about installing some plugs (per Merc bulletin), but we had good water pressure at beginning of last season - so I don't believe that is the root cause.

Did some on water confirmation last night.

I replaced the sea pump again - new housing and impeller.

Just pulled boat off the trailer, in neutral, revved motor to about 2500rpm, and water pressure shot up to about 9-10psi, which seems good! At this time, water temp about 130*, oil temp about 140* - so engine was cold, but warming up to normal operating temp.

In neutral / idle, water pressure gauge needle is basically laying on the pin (0~1 psi).

After idling a bit, we start to get on plane (which I try to not go over 3000rpms during planing). Water pressure is about 3~4psi. After getting on plane, we are running about 55mph ( I think about 3200rpm), and I've still got about 3~4psi water pressure. WTF? After running a mile or so at same rpm, oil temp starts to come up. Gets to about 180* or so, then comes back down to 140-150* - I'm guessing this is an oil thermostat opening. It cycles like this ever so often during our cruise run. Other engine has about 8~9 psi water pressure, roughly same water and oil temp.

I did shoot the thermo housings on both motors with an IR gun, and the water temp gauges are about 10* hotter than the outside of the housings.

Then I make a 90mph pass at 4800~5000 rpms, water pressure at maybe 4~5psi, water temp up around 160*. Didn't look at oil temp, but it wasn't any higher than 180* or so when slowed down.

I'm pretty confused at this point. I can rev in neutral and get what looks to be normal water pressure. But underway, water pressure is low. Could this be something clogged in the drive and/or Hydromotive nosecone??? I called Hydromotive a couple of weeks ago to try and understand the shape/dimensions on the backside of the cone - as I'm wondering if something is stuck in a cavity that could be floating around while underway. ...but they were basically short and rude, and didn't want to discuss. ...so I took it they didn't want to chat/support their products very well.

Also, I've spoken to AZSM (which provided a lot of information/support), and was told that >150* water temp will start to retard the timing. So the engines really need to be running about 120~130* water temp.

Any thoughts? Now what?

Drop the lower, it is quick and easy. You can check both water passages with the lower off. 6 nuts around the drive, then one under the anode (commonly forgot) lower will drop right off with a floor jack to make life easy. Don't lose any of the three orings, two large ones and one small. You will have to drain drive oil first.

Yep - thats what I thought should be my next move, thanks for confirming my logic. I'm due for a drive lube change, probably after this weekend - so I'll probably tear in to it Sunday evening and check early next week.

Pulled lower last night. Was hoping to find a rats nest, but unfortunately/fortunately it's clear. I'm not sure what else to check at this point. Please keep the suggestions coming.

Man, what else is there? Start swapping stuff to the other side?

Every one of these problems I have seen locally has always come down to a kinked hose or a loose clamp. Have you had every cooler off to check for blockage? Flushed block? Changed thermostat? Cut ends of intake hose and reclamped? Interior plastic elbow off to check for blockage and gasket leak? Upper off to check water seal oring?

With dual water pick ups on the drives, I have to wonder if you are sucking air. You could try installing pipe plugs in the upper 2 holes on the side pickups.
The only thing that is strange is why would it start doing that when it didn't do it before.

Maybe

Oil cooler - yes
PS cooler - yes
Fuel cooler - no (was told that it did not have tubes/honeycomb design - 90* elbows prevent looking through)
Flushed block - maybe not good enough, but water flows freely when drain plugs removed.
Thermostat - yes, new
Intake hose - replaced early July
Plastic Elbow - removed and confirmed no corrosion. I did not replace gasket, but applied a small amount of RTV to existing gasket.
Upper - has not been off.

Based on your questions, I will work toward....
Replace plastic elbow gaskets.
Remove thermo/crossover housing and flush block (more)

Appreciate the interest and help!

I thought about that, but why did it start losing pressure?? I guess it won't hurt to try. I could probably wrap some tape around the lower, covering 2 or 3 holes as a quick test.