Epoxy the foam to seal it. You can add sand on the epoxy while it is drying to give it a more natural look.
only thing is that epoxy has a tendency to melt foam. i haven't tested it with expanding foam, but it cuts right through the rigid sheets of blue and pink foam. i think sharkfood is testing to see how it goes.
silicone seems to be the most commonly used method of sealing it. i looked into drylok and cement, but decided against it it ( drylok because of price and cement because of weight and cracking).
if it fails, i can always pull it out and replace it with acrylic sheets that i have handy.