Shallow sea is just that, shallow. It probably is only about knee height. Makes it harder to walk through if you're not a good swimmer, but battle forge is taller than that and the shallow sea isn't high enough to interfere with the magical equipment forging. At least that's how I think of it. As for other hydro weak conjurations...what others do we have again? I know there are creatures that are weak to hydro. I can see how a flame Elemental wouldn't like being in a shallow sea, but he doesn't depend on non magical fuel so it's not like he's bound to a fuel source on the ground. While he can't fly, I imagine that the bottom layer of his body is temporarily quenched while the rest of his body above the water is just fine. And since it's fire it isn't bound by gravity, just its fuel source.
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