[Law considers saying something about how even if this guy is powerful, which he assumes is what the man is implying, he likely wouldn't give the creature indigestion unless he's actively toxic in some way. But then, who knows. Maybe this guy is from a race of people who do generate toxins. Somehow, he doesn't get that impression, but now he's spent too long thinking about this, and should probably address the actual question asked.]

I don't know why I'd want to, it's just water.
[Also, he has no way of knowing if this underground lake is connected to the ocean, and he's not keen on broadcasting his crippling weakness to salt water to a stranger.]

Actually, no, it's not just water. It's water with something dangerous living in it. All the more reason.