In the 18th century British Navy, “Soldiers and sailors typically got one pound of bread a day,” in the form of hard tack, a hard biscuit. That is according to James Townsend. On top of that, some days they had been given peas and on other days a porridge called burgoo. Elsewhere, Townsend shares a some info from a 1796 memoir of the period — the author having written that, “every man and boy born on the books of any of his Majesty’s ships are allowed as following a pound of biscuit bread and a gallon of beer per day” (William Spavens, Memoirs of A Seafaring Life, p. 106). So, grains and more grains, in multiple forms, foods and beverages.
About burgoo, it is a “ground oatmeal boiled up,” as described by Townsend. “Now you wouldn’t necessarily eat that all by itself. Early on, you were given to go with that salt beef fat. So the slush that came to the top when you’re boiling all your salt beef or salt pork. You get all that fat that goes up on top — they would scrape that off, they keep that and give it to you to go with your burgoo. But later on they said maybe that cause scurvy so they let you have some molasses instead.”
They really didn’t understand scurvy at the time. Animal foods, especially fat, would have some vitamin C in it, whereas the oats and molasses had none. They made up for this deficiency later on by adding in cabbage to the sailors’ diet, though not a great choice considering vegetables don’t store well on ships. I’d point out that it’s not that they weren’t getting enough vitamin C, at least for a healthy traditional diet, as they got meat four days a week and even on the other meat-free banyan-days they had some butter and cheese. That would have given them sufficient vitamin C for a low-carb diet, especially with seafood caught along the way.
A high-carb diet, however, is a whole other matter. The amount of carbs and sugar sailors ate daily was quite large. This came about with colonial trade that made grains cheap and widely available, along with the sudden access to sugar from distant sugarcane plantations. Glucose competes with the processing of vitamin C and so requires higher intake of the latter for basic health, specifically to avoid scurvy. A low-carb diet, on the other hand, can avoid scurvy with very little vitamin C since sufficient amounts are in animal foods. Also, a low-carb diet is less inflammatory and so this further decreases the need for antioxidants like vitamin C.
This is why Inuit could eat few plants and immense amounts of meat and fat. They got more vitamin C on a regular basis from seal fat than they did from the meager plant foods they could gather in the short warm period of the far north. But with almost no carbohydrates in the traditional Inuit diet, the requirement for vitamin C was so low as to not be a problem. This is probably the same explanation for why Vikings and Polynesians could travel vast distances across the ocean without getting sick, as they were surely eating mostly fresh seafood and very little, if any, starchy foods.
Unlike protein and fat, carbohydrate is not an essential macronutrient. Yes, carbohydrates provide glucose that the body needs in limited amounts, but through gluceogenesis proteins can be turned into glucose on demand. So, a long sea voyage with zero carbs would never have been a problem.
Sailors in the colonial era ate all of those biscuits, porridge, and peas not because it offered any health value beyond mere survival but because it was cheap food. Those sailors weren’t being fed to have long, healthy lives as labor was cheap and no one cared about them. As soon as a sailor was no longer useful, he would no longer be employed in that profession and he’d find himself among the impoverished masses. For all the health problems of a sailor’s diet, it was better than the alternative of starvation or near starvation that so many others faced.
Grain consumption had been increasing in late feudalism, but peasants still maintained wider variety in their diet through foods they could hunt or gather, not to mention some fresh meat, fat, eggs, and dairy from animals they raised. That all began to change with the enclosure movement. The end of feudal village life and loss of the peasants’ commons was not a pretty picture and did not lead to happy results, as the landless peasants evicted from their homes flooded into the cities where most of them died. The economic desperation made for much cheap labor. Naval sailors with their guaranteed rations, in spite of nutritional deficiencies, were comparably lucky.