Impeller pulls from the tank...
...And is ahead of the discharge fitting. So it doesn't matter to the impeller whether the seacock is open or not. However, the pressure from the build-up of sewage that has no place to go after it leaves the macerator is another matter--blown fittings (big mess in bilge), clogged air valve in the vented loop...sewage forced into the motor housing... Protect the switch from accidentally being turned on, and always make sure the seacock is open before turning it on on purpose.
