Here's a view of the diorama I scratchbuilt for the Model Railroad EXPO several years ago. The Mod Podge water in the rear under the arch was built on a scrap of Styrofoam. The foam is starting to outgas and is ruining the water. The water surface next to the building is okay because it was built on a square of plywood.
Here's an extreme example of failed water on an addition I helped build on an HO Pennsy layout. This is Woodland Scenic Realistic Water poured over a Styrofoam base.
The foam base was painted with acrylic paints and then the water was poured on. In less then a month the bubbling started - small fizzy bubbles at first, and everyday there were more of them. Finally, after after about three months in looked like this - like the La Brea Tar Pits.
From my experience I'd recommend not using any type of acrylic medium over a pink or blue Styrofoam base. I have not had problems using Gator Board.
I have experienced similar results using acrylic craft paint and mod podge gloss medium over white beaded styrofoam also. You've just solved a perplexing mystery! I had guessed that the water was deteriorating due to being in sunlight, or perhaps to moisture changes (moving from dry southern California to WET western Washington). But outgassing fits as the best plausible explanation. Thanks for sharing the failures as well as the successes!
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