The ability to make credits in World of Warships is possible even in a loss. You must play the objectives and choose targets based on threat level and ship priority. Farming one or two big slow ships is not a sound economic strategy nor is ignoring objectives. The following screen shots are prof of this concept.
I can't stress enough how important proper target selection and objective play is when trying to earn credits in World of Warships. If you ignore objectives and farm single ships do not complain if you start losing credits, it's all on you.