It's just a typical 18ft. runabout with inboard 4 cyl 140 mercruiser.
it ahs an inboard tank which I'm really not sure of the total capacity.
maybe 8-10 gal. I'd like to add an extra fuel tank or so, and was
wondering if it's actually legal to do so via coast guard approval.

i was thinking on getting a couple 6-10 gal tanks and putting them
under the rear seats, sealing the vents either tieing them together, or
using one as a reserve., Amd I was planning on filling them with the
main tank. In didn't know if this would present a problem so I thought
I'd ask here.

Thanks