No the current is conducted by the bulk of the steel.
Resistance in the hull is metal resistivity * length / cross-sectional area.
Steel varies in resistance depending on alloy from 10 to 100 * 10^-8 Ohms per square,
I will use an average value of 50 * 10^-8.
Resistance is going to be highest in a circle at the connection point before the cross sectional area gets large
For a distance of only 1cm from the grounding point, on our boat the cross sectional area calculates to
2* 3.14 * 0.6 = 4sq cm (Diameter * PI * thickness)
So the electrical resistance over the first 1cm is 50*10^-8 * 10 / 4 = 125 * 10^-8 ohms or 1.25 Micro ohms
If the negative cable was disconnected and all 10 amps from the pump went through the hull the differential surface voltage 1cm from the connection point is 12.5 Micro Volts. Resistance will go down as the radius increases so this is the highest differential voltage location and 12.5 micro volts is not going to be any source of electrolysis.
Electrolysis is caused by current flowing in the water
. For current to flow from point A to point B there has to be a difference in voltage. If the voltage is zero no current will flow. The voltage on the outside of the hull from either stray or even currents drawn by equipment
is only a few thousandths of a volt spread out over many feet. The incremental voltage along the surface is only 12 micro-volts at the worst location There is no way this is going to cause any electrolysis.
For current to flow to or from the hull you need another electrode to complete the circuit that is at a different voltage than the hull. Just one piece of metal (the hull) in the water
doesn't make a battery until you connect it to a different piece of metal in the same water. The wiring inside the hull cannot provide a circuit for electrolysis since it is not in the water, or even the same water if there is water inside the hull.
A shore power
connection, however, does complete the circuit. The ground lead is connected to the hull on one end and all the shore power
ground at the other end which will be at a different voltage. That is why you need a Galvanic Isolator
in the shore power lead to break that circuit and leave the hull isolated.