Final answer:
Nat Eaton's statement to Kit that 'only the guilty ones stay afloat' refers to the old superstition where guilty individuals would float in water during trials by water, hinting at Kit's innocence.
Step-by-step explanation:
In "The Witch of Blackbird Pond," when Nat Eaton departs, he tells Kit "Remember, only the guilty ones stay afloat." The meaning behind this statement can be best interpreted as a reference to a superstition concerning how guilty individuals float in water.
This idea stems from the old practice of trial by water, where it was believed that the water would accept the innocent by allowing them to sink, but the guilty would be rejected and thus stay afloat.
This line is Nat's ironic way of suggesting that Kit is innocent and therefore should not fear sinking, juxtaposing the absurdity of the superstition against the fear of drowning.