Isn't the density unaffected by the weight the boat is carrying?
The density of the boat is necessarily greater than the density of the material used to make the boat.
The volume of the boat remains the same regardless of whether it's loaded or empty, but its weight is increased by passengers and cargo -- especially when the cargo in question is at least 500kg of gold. A two-passenger rowboat not big enough for a third person probably can't carry that much gold without sinking.
Komachi's pelvis, femurs, and feet are probably ground to dust under all that weight as well.
