I'm not going to tell you what you want to hear, but I think what I say would be echoed by most of the others; that's not the right equipment to attempt a tuna run.
Jets are designed to run at speed. That the ONLY efficient time they run. But the ocean oftentimes will not permit speed and efficiency. Many, many times we've slogged our way back, bow up, motor churning, drinking fuel like a sailor on leave. A jet would be a disaster in that case.
Also, we strive for a rule of thirds; return to port with at least 1/3 fuel in reserve. Most of us have tanks large enough to ensure that. You will have to look long and hard to find a boat on the tuna grounds that carries only 50 gals, and that's with a prop, not a jet.
You will be much better to find a friend who has a boat designed for offshore fishing, or stay with charters. Seriously. Or.... you could buy a bigger boat. :grin:
Now how 'bout that story.
Skein