some sort of nutritional deficiency

Vitamin C is probably the most important one, considering that it was a common issue for age-of-sail sailors - after a couple of weeks at sea, they mostly had hard (probably whole wheat) bread, preserved meat, probably some preserved vegetables that got their vitamin c cooked out of them, water and usually some sort of alcoholic drink, and scorbut is AFAIK the most common nutritional deficiency issue they developed.