Here's a pic of my buddies 90 gallon with a foam wall. Painted with 3-4 colors of marine paint to make it look coraline.
It worked great for a while, but started to break down and expose bare foam. My thoughts are to seal it with a water tight appoxy type marine coating after painting to solidify it. He never kept things like urchins so i don't know how it would have faired against them.