Catalina 30, so either the universal M25 or the 12hp. My buddies m25 came with no strainer - f'in Catalina saving a buck.
If you popped the pump
face plate for the impeller and found no problem you either closed the seacock before hand, in which case you know if it was closed/open before - or if you didn't close the seacock and opened the water pump
and the boat didn't sink then you have a blockage elsewhere before the pump
If the seacock to the raw water is open, close it (handle down tight against the hull). Disconnect the hose leading from the seacock to either the stainer if one is installed, or the pump
if not and check it for flow (blow air through it)- if that hose is clear - quickly crack open the seacock just a fraction and see if water shoots in, then close it. If water came in the thru hull is okay, if not you have a blockage on the outside that needs to be cleared away.
If water came in from the seacock, and the hose from the seacock was clear follow that path to all the way to the pump (including the strainer if one is installed) to ensure that it's open to water flow (blow air if needed to make sure it's open) one step at a time; reassembling as you go.
If you find a blockage clear it or replace the hose with a properly rated replacement hose.
When done DON'T just open the seacock and go, carefully check for leaks and fix them before you go topside to start the engine. If it leaks without the engine running it will gush once the pump cranks up.
If the water path to the pump is all clear it gets more complicated - come back for more input then.