Volvo Penta AQ125A Boggs down under load and dies, milky oil

So made it out to lake yesterday after replacing water pickup, impeller and maintenance items. Engine runs fine and will rev up nice but as soon as I put in gear and go 100yards she bogs down at anything higher than idle speed and will kill... I can tool and pubs at idle speed but anything higher doesn’t work. Checked oil and looks milky froth on dipstick which wasn’t case when I ran it on mufffs as well as milky cream on underside of oil cap. The black 3/4” hose to top of carb seems to be spitting a fair amount of steam so my thought is I need to replace the exhaust manifold as I must have a water leak somewhere, coolant levels look good and engine temp stays good. Any thoughts if this would cause engine to die under load and that I am on the right track?

Update, took exhaust manifold off but everything looks normal, no signs of rust around ports and the ports look dark black char, no signs of one being shined or having water leak.... now I’m confused as I don’t think it’s head gasket. Already have new manifold arriving tommorrow so I’ll replace that. Wouldn’t there be some signs of corrosion or rust if I had a leak in the port?

If not the manifold the oil cooler is another possibility is the coolant level good in the engine?

Yes coolant level is stable

Is there a way to troubleshoot the oil cooler or does one simply replace the O-rings?

Also I’m no mechanic but I don’t see where the oil cooler is on this Aq125a, the schematic shows it by the oil filter but I don’t have any of those cooling tubes or anything on my engine

So I replaced the exhaust manifold and changed oil twice. (Took the plugs out to do compression check and the number 2 cylinder burped some water out so pretty confident that was the issue. I Used this one:

https://www.iboats.com/shop/sierra-18-1930-2-exhaust-manifold.html

the new manifold had a port on the side and came with a big brass plug with another thing attached to it, my old manifold did not have this port on the side. I installed the new one and everything fit good, started the engine and it ran fine however I am getting some smoke in the exhaust and the new exhaust manifold gets really hot. I am wondering if this manifold is the wrong one for my Aq125a? I assumed since I. And with plug that I could just plug that hole? Can anyone offer suggestion?

That is the correct manifold the brass fitting in a drain petcock. You need to drain the water out if it freezes in your area. Sounds like you need to figure out why it is not getting water. Water should flow free out the exhaust shortly after you start the engine.

Gentlemen, So I have 2 questions:
1. please see attached picture of new exhaust manifold, as stated this one has a big brass plug on the side that my old one did not, it does have the brass petcock on the bottom for draining in addition too so not sure if this is for a water heater or what. Additionally the new one does not have a "split" plate at the exhaust plat internally.... the old one had the exhaust split with internal plates and this one does not appear to have any... when engine is running the bottom of exhaust manifold will stay cool but the top get pretty hot darn hot. I am still concerned I do not have the right one? but on to question 2.

2. replaced impeller, water gooseneck piece in outdrive, hoses, O-rings ect. but at idle speed I can take the cap off of my sea water strainer and watch the water pump... it does not pump very fast at all and surges up and down but doesn't overflow with cap off. Obviously when I increase RPM's she pumps faster, overflows ect and at that point cools down the exhaust manifold nicely but I a worried it is not pumping enough at idle speeds and that is causing exhaust manifold to get hot. Is this normal I checked belt tension and that appears fine, as well as replace the rubber key piece on the impeller shaft that connects the two shafts together. Any Suggestions on this one?