Have a safe trip.
24 gallons seems like overkill.
You should be able to sail much of the route.
For grins, let's look at a range of fuel burn rate assumptions: Assuming 12 gal onboard and available....
@ 1/2 Gal/hr motoring for 9 hrs = 4.5 gallons , 7.5 gal reserve
@ 3/4 Gal/hr motoring for 9 hrs = 6.75 gallons , 5.25 gal reserve
@ 1 Gal/hr motoring for 9 hrs = 9 gallons , 3 gal reserve
@ 1 1/4 Gal/hr motoring for 9 hrs = 11.25 gallons , 3/4 gal reserve
@ 1 1/2 Gal/hr motoring for 9 hrs = 13.5 gallons, 1.5 gal deficit
I would think 12 gallons would be adequate assuming you will be able to sail quite a bit of the trip.
It's about 20 nm from the bridge down the river and around the prohibited/ restricted areas to the mouth of West Bay. That looks like a very sailable segment. Once you get into West Bay and the canal you may choose to motor or motor sail . You'll just need to make some assumptions you and the new owner are comfortable with and go have fun.
If not then 18 gals should provide plenty of reserves.