Part of my job involves dismantling washing machines, and those used exclusively on cold or cool washes with low temperature detergents are universally disgusting inside, and smell like a mushroom farm. (Gently peel back the lip of the door seal where it meets the drum and see what you find)
At home, I wouldn't dream of doing it. Having seen what happens, we do at least one 60c wash (towels) per week and one 90c wash (dishcloths etc) per month using powder (Daz etc ) to sanitise the machine and clear / prevent the sludge build up. I don't know how you would do this on a boat using batteries and an inverter, but it is certainly good for the health of the washing machine.
Modern machines are cold fill for a very good reason, it makes them far more efficient. Some new machines only need 1700w heaters because they use so little water to wash, but the heating period is a crucial part of the wash cycle. Adding hot water to a cold fill machine shortens the heating period, but also shortens the wash, reducing its performance.