I don't like it for a couple of reasons:

1. the whole tank has be well above the waterline in order for it to drain via gravity. That means the the head discharge hose to the top of the tank either has to be a rather steep uphill run all the way...or there has a to be loop in it above the tank so that it's only uphill halfway to the tank.Iow, it's sort of like putting the sewer line in the attic of a house. And it's just about impossible to prevent waste or flush water from left sitting in the line.

2. Draining the tank via gravity makes it very hard to prevent an accumulation of sludge in the tank 'cuz it's hard to rinse out.

However, unless you're out to sea at least three miles from the nearest point on the whole US coastline, it's a moot point anyway...'cuz it's illegal to dump a tank in all U.S. waters, only at sea beyond the "3 mile limit."