I recently flew from Cardiff airport to Doha. The lady behind the bar was quite happy to fill my drinking beaker with free tap water, even offering me ice. The airport was not very busy, not sure I would feel comfortable asking during busy periods. However, I did notice a water fountain as I went through to the departure gate. It is just after the steps by the first gate on the right hand side. Why this isn’t in the main departure area I have no idea.