won't stay running when in the water?

I have an 87 Mercruiser 260 she runs fine when sitting on land running off the hose. As soon as I get her in the water the gets rough and will sputter and shut down. I have put new fuel treated with Seafoam in the tank, replaced the water separator fuel filter. I'm picking up the new fuel pump on my way home tonight. I removed the exhaust risers and manifolds yesterday and cleaned them up ( the risers had a lot of Gunk) to reinstall after the new fuel pump.
Does anyone have ideas what might be causing the problem? I'm running out of ideas! It just throws me that it only happens when in the water.

Thanks!

Is it safe to assume (!) that "...as soon a I put her in the water..." means that you are talking about at idle and not in gear????

It happens when I bring the rpm up. acts like she is starving for fuel. On the hose at home no problem.

Because at home the boat is not under any kind of load. You def have a fuel delivery problem. Check on obvious stuff like the accelerator pump, and that the fuel hoses are free of obstruction both before and after the filter.

and don't forget the anti-syphon valve, especially if its original!!

Remember to check the vent too. I've seen/experienced home(s) made in the vent hoses (usually appears at season start-up). I think I'll replace my anti-syphon "check" valve next season based on makomark's comment...