2002 115 EFI 4-Stroke / Water in fuel, HELP!!!

[02-115-Merc]

I had a bad issue with getting water in my fuel. My throttle response was bad. It got to the point that it would stall out when you gave it gas to fast.... I've since installed a fuel filter/water separator and took the boat in for service. They cleaned the entire fuel system and now the throttle response is back, but I'm loosing RPM's as I go. For Example: I give it 3/4 throttle get it up on plane at 3800 RPM and it slowly falls to 2800 RPM, If I throttle back a little then throttle back up it picks up RPM's again, but then it starts all over again.... Cruising speed used to be at 42 MPH at 4300 RPM, now I'm lucky to get 25-28 MPH at 3800 RPM.

Any suggestions before I take it back to the shop?

[boats.net]

Need to check to see if loosing fuel pressure at high rpm's.

[02-115-Merc]

If that is the case... is it just as easy as replacing the fuel pump.... Also could this be happening due to bad gas?