Coring is often used for weight saving. Plywood makes a fine core, but it is much heavier than balsa. Balsa usually comes as squares of end grain wood, properly done, each square is supposed to be isolated by resin, which limits water spreading. Unrotted balsa has very low water penetration across the grain. These two things are probably the origin of the idea that balsa is less prone to water seepage and spread. In my (painful) experience, this ain't so in the real world. Plywood gets softer and softer as it rots, eventually is mush with no strength, but it takes years. Balsa vanishes when it rots and it doesn't take long. Being end grain may help a little, but not much.
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