Been watching this thread ........... here is a tip out of my secret tuning archives....
Spray bars work very well in B Headpipes, however, location is critical! In the early days of the ECWI, they recommended installing the spray bar in the mounting flange. They did this mainly for simplicity as the inner waterjacket did not have to be penetrated for the spray end of the nozzle to make it into the Hot Gas Path.
In reality, performance gain was acceptable, but far from optimal. The other issue it presented, was if it had a leaky solenoid, water found it way back into the motor very easily. Thus, quite a few motors were lost to water injestion. The top area where the OP is showing, will work fine for a rec boat with small displacement motor . What you need to do is remove the head-pipe from your ski, and with a 1” hole saw drill thru the outer water jacket only!! Then have it welded up as in this attached photo.
Once complete, drill and tap the inner chamber wall for the spraybar. The spraybar will also need to be tapped to accept Mikuni main jets. Install with the spray pattern going with exhaust flow. The line feeding the spray bar needs to be T’d into the cooling water line feeding the Exhaust Manifold. You want cold water at the spray bar, not hot! Make sure you also use some type of filter before the spraybar as the small jet will clog easily. For tuning. Close all std FPP adjusting screws. Start with a 100 main jet in the spraybar. Do not go smaller than a 70, and you will more than likely not go larger than a 120 Tune to your desired powerband. Lets us know how you make out?
Ski ya, Paul