It all depends on how you are prepping the steel. If you sandblast you'll need approx. 4 gallons of
RustSeal. Each gallon will cover 200 square feet two coats. By the dimensions you've given us, each drum is about 240 sq. feet each.
Since this is a marine application, we recommend three coats especially from just above the water-line on down. The four gallons would allow you to brush, spray or roll on three coats on both complete drums.
If you are not sandblasting, I would recommend the following:
2 - Gallons AquaKlean
3 - Gallons RustBlast
4 - Gallons RustSeal