Clean and descale all water passages including thru hull and ensure the pump is operating to spec. Seaflush or Rydlyme are several of the chemicals/methods you can use.
In the short term, sometimes you can clear enough out with pressure water from the dock... at least to get you through until you can do a full descaling.
But to cover some more bases (if you haven't already), also check for obstructions at the inlet and in the sea strainer... and that the water pump is primed and working, etc.
Sometimes it's as simple as a plastic bag covering the intake thruhull. Turn it off for a few minutes, then turn it back on and see if that cleared it. If not, proceed with the other suggestions.