OK, made some progress, here's where we're at:
I was able to reach the plug in the pan. drained out a couple more gallons of water and then the oil which was completely milkshake. Towards the end of it the was definitely noticeable pink stuff: antifreeze.
I pulled the port the plugs. #3 poured water (matching #4 which had been full on the other side).

Note that after extensive fooling with the stats to get the right temps, I removed them last august after trying a lot of different options, so a closed stat didnt stop antifreeze from getting in the block.

Now something I hadn't gotten around to mentioning was that when I winterized last November, the starboard oil did look a little bit milkier than the port. I wasn't really sure and also thought at the time it was because of the cold water temps with no thermostat, and the motors not being exactly the same (when the motors were put in the boat in the spring, the starboard also had more milkshake until it warmed up in the summer). So I dont know if this is related or just a coincidence.

Whatever has gone wrong yesterday, if it was already an issue in the fall then it was much less so, since it seems that running the motor for about a minute caused the entire amount of oil to froth up (there was no normal looking oil in the motor).

Im heading out now to Walmart to buy a bunch of cheap oil. What should I do though considering that there is an obvious major leak inside the motor? Fill it with oil and run it for 30 seconds without raw water? Im assuming I need to run the engine to get the old milkshake dislodged and down into the pan.

Let me know what you think gents, your help is greatly appreciated here.

30 sec with no water to the sea pump will most likely smoke the impeller,

Take belt off sea pump or remove pump completely. Needs seals or impeller or checked out anyway. Crank motor to blow water out of pistons. Add 1 quart of oil. Crank motor looking at oil pressure gauge. You get pressure with 1 quart? Put in plugs and run it for 1 minute. Drain fill repeat.i cut a flat drain pan from a milk jug. Usually you can make a 2 quart pan that will fit. The closer to true empty it drains, the less times it will take to get watery oil out. NOW pressure test and start taking parts off. Cuz if you pull the intake, followed by the heads, it may be weeks before it goes together and gets run up to temperature. So you don't want oil system and bores sitting with watery oil. But just use the least oil you can use to get oil pressure.

If he tries to run the engine on 1 quart of oil he will be rebuilding it for sure, you're in good hands OP......Im out

These Merc 502's were fully rebuilt when they went in last year although the seller definitely misrepresented them (I spent many thousands replacing **** rotted out accessories) and I now judge the engine builder as somewhat dubious base on things he's communicated to me and things which needed correcting on the motors. I am doubtful about rotten intakes as I needed to replace one straight off before it went in the boat (the other one was fine).

I got a bunch of cheap oil at Walmart, will put 2.5 per run and keep an eye on pressure. Thanks

<div style="text-align:left;"><br /><br />I agree. What exhaust on boat? I'd wanna be pulling and digging into engine asap personally. Just sounds like that engine been an issue from day one. At the very least I suppose pressurize the system with air. Very easy to do right through the bypass with air and cap off the exhaust. Not a lot of pressure, and just listen with dipstick out. You could actually thread a air chuck fitting into bypass with adapter and pressure gauge. Multiple ways to check. <br /><br />A leak down after the fact would be next if it were me. Next prepare to pull engine worse case but might be further ahead if this engine been an issue from day one. Doesn't take long for water to start doing damage internally. Fresh or salt. </div>

Stainless marine. And im pretty sure they aren't the problem. I filled up the water jackets, not under pressure but they didnt leak at all, and I had only run the boat a couple minutes max, no time for them to get really hot.

Right now im just first worried about getting the water out of the motor. Got the risers and manifolds back on and about to start running clean oil through it but had to come in to warm up. Winds are around 30 mph its damn cold out there on the dock.

And yeah, I would really like to have those motors apart. Gonna have to see whats in the cards, after what I threw into repowering the last thing I needed or hoped for was to have to do it again. It would have cost me less to buy new engines after it was all said and done. If this wasn't my dad's boat I'd probably be giving up on it by now. Im also 70 miles by water from my engines guy, dont have a trailer or tow vehicle, boats on a lift out back. But I'll worry about that later, just need to get them dry inside now.

Alright, had my coffee, now back to the bilge!