If it's truly a dry water-jacketed header (all the way through the end of the collector), you *might* be able to drill through both walls in the collector and weld a bung in , assuming you have sufficient access to the inside of the collector to make that weld. But the pipe is going to run cold, adn O2 sensors like to run hot.
I have tried using an O2 sensor in a short section of dry, "uncooled" pipe (just aft of my logs) as a tuning aid for my carburetted engines, but have trouble getting any kind of useful life out of the O2 sensor before they crap out.
An O2 sensor implies that you're intending to use a feedback fuel injection system. Not sure if that's really necessary (or desirable) on a jet boat, since the purpose of a feedback system is to keep the F/A ratio near stoichiometric during cruise conditions, for proper function of the catalyst system in a car/truck.
If you go with an open-loop (pre-mapped) system, you don't need an O2 sensor.
But I'm no expert on either one of them.
