Ooh, this is a tough one. Trying to remember as I did watch many shows as a kid but some I know of in hindsight or from parents mentioning it rather than my own core memory.

I think it might have been an episode of Aladdin the Animated Series, although I know for certain I watched other things than that before, just can't remember.

It was the episode with the mermaid witch who tries to steal Aladdin away from Jazmine as she's trapped in a water dimension and can only witness it all from other clear water surfaces. Can't remember what I thought of it as a kid but it's the oldest one I can personally think off, especially since I don't think I've ever rewatched the episode since then.