is best described as closed cell rubber foam - or small cavities formed by gas bubbles in rubber.
Closed cell - there is no gas exchange between bubbles. Tends to be quite water proof.
Eventually, due to use and abuse that closed cell foam can become open cell foam - some of the walls will rupture, and the open cell foam is not that water proof.
Perhaps you could stick those boots in the water, blow some air and see where the bubbles are originating?
Kayak Motor Kit
Deck Rigging Gear
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