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.