The RNLI advise against running an engine for more than 2 minutes on muffs. It can cause the engine to oil up and the plugs to foul. For long periods use a tank so that you get the correct exhaust back pressure. It will also give a truer idea of the health of your water pump.2 and 4 stroke - Applicable for overwintering \Servicing
Run the engine on muffs for at least 5 mins to clean out salt and to warm it up.
A poor telltale output may not mean a sick pump. Could just be a bit of cack - I once removed some earwigs from an engine whos owner had replaced then refitted (twice) a water pump impeller when the old one was fine!