Yes I mentioned this several posts ago, low demand, all good. Increase demand and suction and flow go up unless it can draw air in someplace and if all the hoses were off at some point its a very real possibility.
You could always buy a length of hose and swap lines from engines and see if issue follows to other side or remains on the same engine . That would either pinpoint or eliminate the suction side of the system.

Did you check the clamps on the hose at the waterneck? (opposite of sea pump).

You might also put a wrench on the bolts at that waterneck to ensure they are tight - they are small though, so don't break them off. Oh wait, you have transom pickups don't you? Nevermind.

So all the water pressure from everywhere looks more then great. I should now try to tighten every Hose to see if maybe I have an air leak. Starting with the sea strainer

If it's a pressure problem crimping the line going initial the heat exchanger will give you a better reading if the sensor is before the heat exhcanger. If your low on pressure your either don't have the volume to create the pressure or you aren't restricting it anywhere. How does the temp of the motor act. Of it never gets warm because the stat is stuck open that may cause it. Or even if you have a water pump and it isn't spinning anymore. If it's not sealing before your impeller you will suck air and not get enough water. If it's after the impeller you would see it leaking. Think suction and pressure on your impeller. Verify every hose is strong and clamped tight before the impeller and water pump including the lid on the sea strainer. I have seen where your water pump socks the hose shut going to it.

You are in Pittsburgh you should just come take a look haha.. The temp on the motors are great, and only a degree apart really. They never get hot, and neither does the exhaust for that matter. The Impeller, and all the gaskets have been replaced. It has to be an air issue now. Just not sure which one, and to be honest im fricken fed up lmao

It's Dave with the black and gold donzi 22. I can come check it out tomorrow night if you want another set of eyes

This was today on plane at about idk maybe 3500 RPM http://i47.photobucket.com/albums/f1...7/IMG_7102.jpg

Oh hey Dave. Im bad with remembering people through there screen names. Im pulling the boat out of the water 2marro at 7 regardless of anything. Im leaving for Deep creek lake friday so I have to get my poontoon out, and cleaned as well.

Not a big deal. I just know boats as well. But what Lenny has done in the past is bought 1 1/4" clear vinyl tubing to put in place of some hoses this way you can see the amount of water

Replacing hoses on 600sci's is no easy task. Especially when they are beside each other. Some yes, but others no. Im going to have my mechanic try to tighten up all of the hoses 2marro, and see whats up. Either way im going to pull it out fixed, or still sufficating tomorrow evening. Just trying to find any other leads while hes down there. I have heard that the Intercooler can throw curve balls on this cooling system as well

I think the thermostats on the Mercury closed cooling systems are on the antifreeze side of the motor. I know they are on the 496s.

^^^^^ Did not read any posts here but Brians above so will add the - 600 SCi is somewhat the same set up as the 496 for the front side coolant end. The t stat sits in the port side crossover with the heat exchanger mounted on top of that. The crossover is the anti freeze side as well. T stat's and water circ pumps still go bad in anti freeze sides regardless especially if you never change the coolant to new. No one likes to drain the complete system of anti freeze on Merc closed cooled systems. The coolant depletes its anti corrosive properties overtime and from heat and cooling cycles. It needs to be changed .

More info o top of - the t stat here starts to open at 143 degrees and is fully opened 100 % at 158 to 160 degrees. Anti freeze capacity should be 4 gallons total.

Ok so we finally got to the fuel cooler, and back flushed it. Bunch of debris came out of it. Took the boat for a spin, and its 90% better. So close.... Must be one more chunk of something caught in it.

So the fuel cooler is not a wide open style? It has the holes/grid/ or tubes through the cooler?

Yes it has the grid through it.