Being the resident “NOT EXPERT,” a couple things come to mind that could have caused this, a lot of air, or moisture.

When I applied thin epoxy coats, I had to seal the wood to keep the air from moving into the resin before it dried. Even sealed, some got in and I had to address it. All that said, to turn milky suggest one heck of a lot of air, so unless it shows the tell tail signs of a bazillion air voids, it’s probably something else.

Of course, if the temp was really cool and the viscosity high, it might be the natural bleed off, when used at the right temp, could play somewhere.

I’ve never tried mixing epoxy and water, or applying it over water, but I’m wondering in that direction.

What are your thoughts?