Double check your work here since this is when the backfiring started.

You said it has been falling on it's face at wot ever since you have owned the boat. Correct? I had this happen to me right after a total rebuild. The lobe on the cam for the fuel pump wore off in less than 30min of run time after 1hour and 45min. of break in. My motor had the same symptoms you are talking about. I don't know how to tell you to check this maybe mercman can help in checking this.

Install a in line, fuel pressure gauge at the carb.
Run it without the hatch if possible and have someone monitor it at WOT. If it starts falling, ya know what it is.


Darrell.

If the cam lobe for the fuel pump is worn wouldnt this condition exist at all rpms? Also, does anyone know what kind of pressure I should have on the fuel line at the carb?

5-7 psi, preferably the 7psi.

My suggestion on the first thing to do is pull the valve covers and check the pushrods and rockers. Unfortunatly, I've got alot of experience with this.

If the cam "pump" lobe is worn, it will cause a reduced movement in the fuel pump, reducing volume. At lower rpm, it may be "enough" flow for the motor. As demand increases, the pump simply cannot keep up. Just one possible cause.
I chased a backfire last year in my ski boat. Went through the same checks as you. It ended up being ONE plugged pasageway in the carb. After about a month of pulling my hair out, I was able to blow it out with compressed air....and was good to go. Total PITA.


Darrell.