Inside length, width, and average depth in feet—US gallons (1 cu ft ≈ 7.48052 gal).
Length, width, and depth are treated as feet to form a rectangular volume in cubic feet, then multiplied by 7.48052 (US gal per cubic foot in code) to get US liquid gallons (one decimal in the output). All three dimensions must be positive; comma decimals are accepted.
Steps, benches, and freeboard are not modeled—only a simple rectangular prism in feet.