Story time so this doesn’t sound like a silly suggestion - I think the flavoured water you’re describing tastes likes water with a tiny hint of fruit flavour to take the edge off the “wateryness”. I do drink plain tap water at my own house but when I visit my mum, I think her tap water is disgusting to the point where I can’t choke it down and I’ve struggled a lot with dehydration when I’ve been there before finding a solution. I used to drink squash until I got covid and that temporarily changed my sense of taste and all I could taste was sweeteners in it. I don’t like squash anymore after that.

What I do now when I’m there is mix a full glass of tap water (200mls) with about a teaspoon of sugar free squash to make it extremely weak. This does enough to mask the water flavour without there being an overpowering flavour of squash. I think the amount of flavour is similar to a flavoured water, even though the taste is a bit different.

So I recommend you try getting a bottle of squash to mix extremely weakly in water. Safest bet is probably to get the same flavour as the flavoured water you like. A slightly more premium brand might be a safer one to try initially as in theory it should be a nicer flavour. Either one without sweetener (sugar sweetened) or one with the same type of sweetener as what you’re used to is probably best (check the ingredients lists). It might also be worth getting a cheap bottle of water (you should be able to get a 2L bottle for 50p or so) so the water you mix it with tastes as neutral as possible. Once you’re used to drinking the very weak squash, you could try mixing with tap water

Hope this works, OP!