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I’d imagine the amount of energy you’d have to push into the water would be insane to hear it 1500 miles away, wouldn’t it?

Whale noises travel thousands of kilometers underwater. https://en.wikipedia.org/wiki/Whale_vocalization

There's an inverse relationship between wavelength and range, and the higher the frequency the better the resolution.

Pour in some traceable mix of chemicals, and see if it turns up somewhere.

There're these fish called the Devil's Hole Pupfish. Can't do that.

There is a near infinite number of non toxic chemicals to choose from.

Ah, but can you show that they're not toxic to Devil's Hole Pupfish? And the population is tiny and fragile--you're not allowed any test subjects.

There might well be 5 separate regulations forbidding it. That's a different question.

The safest class of substances I can think of are unusual isotopes.

They're chemically identical to the regular elements, which is what matters for life forms, but can be detected by scientific instruments.

What other end?

You'd deafen the poor troglobites.