A friend of mine has a 7.4mpi and the other day we went out for a ride stopped, took a dip, then fired it back up to go cruise again. After we fired it back up it ran real ruff and spitted and sputtered. The mechanic he took it to said the cool fuel system needs to be replace, he said that it was leaking down causing air to get into the system. I figured I ask yalls opinion before he spent the big money to fix it.

It's more than likely vapor locking, especially with the heat you guys are having. Merc's efi engines are known well for it.
He has 2 options to fix it:
First it to return the fuel back to the fuel tank instead of the fuel filter/water seperator head. Right now, the fuel is running around in a loop and getting hot. It's boiling in the line and that is what is causing the air. Returning the fuel to the tank is an absolute cure for it. It depends on the accessability as to if this is the best route.
Second is to install the update kit from Merc. They place an electric pump in-line between the tank and the filter. This also seems to work really well.
The only thing in the cool fuel module is the pump and a cooler. If the pump is running, then there is a 99.9% chance that it is good. Generally, when they fail, they quite running.(there are a few exceptions) Obviously, his is pumping fuel if the boat runs when it's cool. The other is the cooler. If it is leaking, then it probably wouldn't run. The gas would just get pumped into the water hose.
Basically, what I'm saying is that replacing the cool fuel module WILL NOT fix it.
Good luck.
Eddie

Most likely vapor locked as Eddie said. Its a common problem with the Cool Fuel system. Merc has a TSB on it and their solutuion is the booster pump kit.

I did leave out the boat is twin, is it common if only one vapor locks and not the other? Also if you were to run a return back to the tank would you have to take it and get a bung welded into it or can it be done where the sending unit comes out of the tank? Is there one cool fuel for both engines or is there one per engine? Sorry for all the questions but my friend is one of those just do it kind of people and I don't want him getting ripped off. Thanks for the help! Oso rules!

I keep seeing threads where people are putting returns back to the tank. I thought that was against coast guard regulations? Isn't that the whole reason the Merc fuel injection system was designed the way it was?

Not, they did it for packaging. The engine is a self contained unit and easier to install for boat manufacturers.

T-Rav, there is one cool fuel module per engine. It is not terribly uncommon for one to do it and not the other. It depends on how the fuel lines are run. One may be closer to a heat source then the other.
As for the return, you have a few options. You can return the fuel back to the fill hose with an adapter. You could also make a bolt in return. If you do a search in this forum, I just described it for another member recently.
The easiest, but more expensive, fix is the update kit from Merc.
Eddie

Thanks a lot I will try the search I appreciate the answers.

I have 2 of the Mercruiser kits available, primer pump bracket line wiring filter, brand new.

Usually the port engine will vapor lock easier since the fuel filter is near the exhaust manifold for both engines.

diesel boats have a sending unit that has a return fitting already in it...easy way to add a return