Correct, don't use RO or distilled water for yeast rehydration. It'll actually kill cells. You need to use water that's osmotically balanced with the internal yeast environment. Bottled spring or tap water works fine, and is, for all intents and purposes, sterile. You can use tap water (assuming it's from a municipal water supply, not a well), which is also, for all intents and purposes, also sterile. But your sink faucet and first few inches of sink piping are likely not, which is why tap water isn't ideal.