where to from here ? ..... slip boat out of the water,.... replace seacock . and skin fitting if required.

oh !! and yes water should rush in the inlet if the tap is open so its not opening or blocked, i had similar problem .... then the handle broke off and ..... it was not turning on.

I had a similar problem with my 28hp Volvo, it turned out to be the exhaust elbow was corroded and would not allow water to flow out.
We tried a acid bath for two days to remove the corrosion but without success.
Had to buy a new one, not difficult to fit but use gaset grease to seal between elbow and engine head.

You can check if it is the exhaust elbow by disconnecting the water line that enters the elbow and connecting it to a length of hose and then into a bucket.
Run the engine and if water flows into the bucket it must be the elbow.

Then how come? you open valve and water doesn't come in??

To answer about water coming into seacock.
I think seacock is open but the height of seacock say 6 inches above hull is not enough to let water flow through. Inlet is not very deep below surface thus there is not much pressure. It needs the impeller not the force of the water from outside the boat to get water to the motor.
Once elbow was cleared, happy days, heaps of pressure.