Hi Kevart

Is the boat out of the water? If so, disconnect the hose inside and run a stick down through the seacock. Also inspect from the outside--you can sometimes suck something up.

To check the pump, close the seacock then remove the pump. If a visual inspection reveals no clogs on the intake/outake, apply power with jumper wires and a battery. If the pump isnt clogged, but powers up, it probably needs to be rebuilt. Rebuild kits are available for most pumps from the pump manufacturer and retail stores like Hamilton Marine.Here's an example http://store.hamiltonmarine.com/browse.cfm/repair-kit-crown-head-standar...

Good Luck

Kevin