Sometimes a plastic bag or weeds can cover, or get sucked into the raw water intake. A quick dive with a coat hanger hook can check if there something in that thru hull. Then close the thru hull and open the impeller cover to check if all the vanes are visible, none missing, and no visible cracks in the impeller. If ok so far, open and shut the intake seacock to see if water comes thru to the impeller opening. If not, there is still a blockage there. If yes, the blockage is further up the system towards the heart exchanger.
I'm assuming you have enough coolant in the engine. Also, on our boat, and others, you can get an air lock in the cooling system, requiring bleeding thru the pet cock near the thermostat. In fact, I would check that first, and if you get air bubbles coming from the petcock, there may not be a blockage elsewhere. To check for bubbles on our boat I remove the cap on the overflow tank, then open the petcock, with a rag in hand.
Let us know how you make out.
Frank