Huck augments their diet by buying things at night at stealing things from shore.
As Huck and Jim are traveling down the river, food is scarce. They do all right hunting and fishing, but sometimes that’s not enough. Even the novelty of answering to no one and being on the run wears off.
Eating only water fowl and fish day in and day out would get boring, and not healthy besides. At night, Huck visits the village and buys additional food. He also sneaks on shore, and visits local farmers as they pass by.
Mornings before daylight I slipped into corn-fields and borrowed a watermelon, or a mushmelon, or a punkin, or some new corn, or things of that kind. (ch 12)
Huck describes how his father did not consider it stealing to take food that you needed. It is an example of Huck’s continual exploration of morality, as he tries to decide what is right and wrong.
Ah, Jim's organic diet! Your question made me smile a bit. Huckleberry wasn't good with only organic food. How does Huck fix that? Stealing. Let's look at the specifics in the text before we explore the reasoning behind Huck's decision.
... I slipped into corn-fields and borrowed a watermelon, or a mushmelon, or a punkin, or some new corn, or things of that kind.
Of course, Huck does these things "mornings before daylight" in order not to be caught. I find it interesting that Huck, probably without realizing it, has gotten their sustenance more into the realm of the "four food groups." He has added starch and fruit to Jim's suggestion of pure protein. The only think that Twain leaves out here is dairy. (And my guess is, Huck went in there and milked a cow or two if he had "a hankerin'.")
In addition to the politically correct food group reasoning, food is simply scarce where Huck and Jim are hiding out on the Mississippi. Note that Huck never uses the word "stealing" himself, ... he uses the word "borrowed." And, to his credit, when he has the money, he does buy food sometimes from local towns.
In conclusion, it's important to note Huck's moral compass here. Huck's Pap has always taught that taking food that is needed isn't wrong. Huck's sense of morality is a big focus of this novel. In fact, this leads us to his big conundrum when he has to decide whether to turn Jim (a runaway slave) in to his owners. His decision? Not to and, as a result, to "go to hell."