Take the hose off the outlet of the pump. You may or may not have the solution shortly.
Closing the seacock before you remove the hose might not be a bad idea. After removing the hose, open the seacock to see if you have flow. If there is flow, close the seacock and energize the pump (it will be a small mess).
If the pump splashes water, there likely is a blockage after the pump.
If no splash, probably the pump.
If no water comes out when you open the seacock, there may be a blockage before the pump or the boat isn't low enough in the water.
These pumps are generally centrifugal and many are magnetically driven. Centrifugal means they must be below water level to prime. Magnetically driven means that some thing can block the impeller from turning even though the motor is running.
If water comes out with the seacock open but the pump doesn't make it splash, turn the pump off and try to spin the impeller by sticking a screwdriver in the outlet and flicking the impeller to turn it.
If you find that the blockage is after the pump, check the discharge outlets. Mud wasps love to use them to build homes.
Ted