Made from what? Are you talking about a full illicit synthesis of GHB? Or GHB made from GBL?
The answer to your question is almost certainly no. The way you worded your question suggests that the ‘someone’ who told you that probably wasn’t a chemist or someone capable of performing a multi-step synthesis of GHB. It was most likely about the synthesis of GHB from GBL, which is a relatively straightforward process. However, if you’re going to isolate the GHB as a salt, it would be better to use distilled water to avoid the extra minerals found in tap water.
Either way, I can't think of a reason why using tap water would specifically make the product more harmful. Unless it's very shitty tap water, something that would be harmful to drink.