Hey so I have two stage 3 mercury 700s in a 36 supercat, they run great for the most part. I was getting a low fuel pressure issue anything after 4k rpms. Turns out I was getting water in my fuel filters and this was causing the issue. I had both tanks drained, new filters etc took the boat out got roughly 120 out of the boat at 5700ish rpms then the code came back. Check the filters in an empty bottle and what do you know water in both filters. Swapped again and same issue, ran it again and had the same issue. I went home and drained my tanks again, luckily I just had about 50 gallons for test reasons. Fast forward throughout the summer and this happened every trip. Checked the fuel at the fuel station i use and zero issues there. Am I just collecting residual from the bottoms of these tanks because they are coffin shaped or what could the issue be? Left the boat at a shop originally and left with the same issue after a decent bill. Next step is hauling the boat out to performance or Teague, long haul though being in Oklahoma. Where could the water be coming from? Anyone else had this issue? Anything I should check etc. Water pick ups are on the bottom of the boat as well. Just stumped and tired of dealing with it. Thanks for any help.

Are you running fuel coolers ?

Are you sure you drained the tanks completely? On mine, I need to jack the bow up high so fuel (and any water) runs to the rear of the tank where the fuel pick-up tubes are located.

Do you have access to the tanks fuel-level sender-unit? If so, you can pull it out and have a large hole to look inside the tank.

I pulled off this hose on the other side of the ball valve, the original shop advised me to not pull the sender because “Do not try to remove the tank sender and pull out of that chamber. It has baffles and that work how you think. The chamber will continue to refill and I don’t want to alter the glued on gasket at the sender”. But maybe that’s my issue, I did not jack it up, had it on flat ground. They drained it and so did I. The fuel I collected in 5 gallon jugs looked great, no water.

F-2 Speedy it sounds awful but I would have to check. I’ve only had the boat for a little over once season so I’m not positive.

quick search and yes

yes, the Mercury 700 SCi (Supercharged) engine does use a fuel cooler, integrated as part of its OEM fuel system, often alongside the fuel pump,

Yes sir, you’re correct. I just checked

A friend of mine did not put a loop in the fuel tank vent and got water sprayed up to the vent during rough water boating and got water in the tank that way, maybe check the fuel tank vent hose can water spray into it while underway ?

The vent is definitely a good guess. I would think water would only end up in the tank from cool fuel if there is a return line. If no return line then I would think the motor would just be burning the water/fuel mix

you cant dead head efi there is a return off the back side of the regulator to the tank or filter head