1st - unload as much weight as possible. Use extreme prejudice. There is probably at least 500 lb. or more of junk you could leave at the dock. Not just the small stuff either. Anything and everything you can't use for the trip.
2nd- tune up the engines, advance the timing (if possible), fresh wires plugs and ignition, and rebuild the carbs.
3rd - Flowscan. Until you have a floscan, you are basically in the dark. If no flowscan, figure out where the secondaries open up (not what your buddy with a camaro says, but what you can actually see with air filter off while operating the boat). Cruise just below that rpm.
4th - Check the props and make sure the bottom is clean. Couple years of wear on the prop or a fouled bottom can cost you speed (and speed at the same fuel burn is milage).
5th - Bladder. PM me if you need a 100 gal. version. I have one I no longer need.
6th - turn off your engines while offshore and stop the long warm up at the dock. If you are that concerned that the engines will not restart off shore, stay at the dock. Of course, don't turn off the engines if you are up current of a rig or structure.
7th - autopilot.
If you are getting close to .66 nmper gal., sounds like you are doing reasonably well for gas at that size.