fuel pressure problem????

I have a 2000 centurion wake board boat with a 350 mag mpi and v-drive. Engine serial # OL394367. The engine started missing and popping back through the the throttle body when you try to accelerate. I changed the plugs (they were white, real white) and I also changed the distributor cap and rotor. This stopped the missing but the popping is still there. When I try to give it gas, it acts like something is holding it back and it will not go to full throttle. I checked the fuel pressure at the rail and it had 18 psi. I replaced the fuel pump and cooler (part # 861156A02). This did not help. I replaced the fuel pressure regulator( part # 807952A1). This brought the pressure up to 28psi. One book I have says the pressure should be 34 to 38, another book I have says 42. 28psi is still not enough. I took ALL of the fuel lines off, from the tank to the fuel rail and blew them out with a compressor. All of them were clear and nothing came out of any of them except clean fuel. I also blew out the fuel rail. The anti-siphon valve was also fine. I installed a new relay and checked voltage to the pump, all checked fine. Timing is also fine. The boat will go to full throttle now with the new pump and regulator (still only 28psi) but if you put it under a heavy load or try to pull a wakeboarder out of the water it pops through the throttle body. The 28psi is when the boat is idleing. I can put a hand operated vacuum pump on the vacuum hose going to the fuel pressure regulator and pull a vacuum on it, the pressure drops. I can unhook the vacuum hose from the throttle body and plug the throttle body and run the boat, the popping is not as bad. So after all of this,,,,, WTF...???? help !!!!!!!!!!!!! Also, I replaced the fuel filter, dumped the old filter into a bucket and there was 2 drops of water in the bottom of the bucket.

I had the exact same problem in my 2003 avalanche. It turned out to be the low pressure fuel pump. Which fuel pump did you replace?

This motor has cool fuel.....

There is only one fuel pump on this motor. Right beside it is a little heat exchanger that "cools" the fuel.

How sure are you of that? I have the exact same motor. One fuel pump takes fuel from the tank and gives it a little pressure and get's it to the high pressure pump. My boat this exact same thing. It took lots of time to figure out what was wrong.

99.9%.....

"I took ALL of the fuel lines off, from the tank to the fuel rail and blew them out with a compressor. All of them were clear and nothing came out of any of them except clean fuel. "

Maybe leaking O-rings on the fuel rail? The gas around here went to 10% ethanol about a year ago. I fully expect to have to reseal my whole fuel system soon. I replaced the fuel lines preventatively last winter.

I had a very similar problem with my 99 Sanger with a 350 Mag MPI. Mine only has one fuel pump. I have a factory service manual that specifies 34-38 psi. It specifies a 5psi drop with 10" of vacuum applied to the regulator. My pump quit working over the winter and boat wouldn't start this spring. I replaced my pump with a Mallory pump. I didn't want to spend $400+ to buy the pump/cooler assembly from Mercruiser. After I replaced it, I had the exact same issue you're having. I checked all the stuff you checked. I called the Mallory tech dept., he had me check a few things. He said the pumps are very voltage sensative, even like .5 volt drop can cause a loss of pressure. It turned out that I just had a bad pump (the brand new one). He said that is fairly common. He also said that the Malloy and Mercruiser pumps are both manufactured by Airtex.

Other possible causes might be a leaking fuel injector, but you'd probably have 1 black spark plug.

Cracked siphon tube in fuel tank, causing you to suck some air. Can you connect a hose from your filter inlet to a small gas can?

Have you checked for codes, mine has a fuel pressure sensor, and it set a low fuel pressure code.

Also try pinching off the return line past the regulator and see if your pressure goes up.

Is it running lean?