Length of wire as well as gauge matter. Houses typically require longer runs than a boat.
Add up the total wattage that wire must supply when every load us turned on at the same time. Divide the total wattage number by 110 to get the total maximum amperes the wire will have to carry. Then Google wire gauge required for that amperage.
Wire has a certain amount of resistance per foot. That resistance times the maximum current is the wattage lost through heating the wire. Heavier gauge wire offers less resistance, therefore less heat loss.
Example: total length in feet times resistance per foot times total amperes equals total voltage drop.
20 feet times .001 ohm per foot times 30 amperes = 0.6 volts
Volts = Resistance times Amperes
Watts = Amperes times Volts
30 amperes x 0.6 volts = 1.8 watts of energy lost in the wire.
Use this chart to determine your total resistance.
https://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table