Do we just pour water in it? I wasn’t sure how to do it if It wasn’t in the water to pull from?
Ah, yes. That is an issue.
A good thing to do regardless of your contemplated on-land scenario is to tee a line from the head sink into the salt water intake line for the head. That way, you can run fresh water through the pump by keeping the thru-hull closed, filling the sink with water, and using the pump on the head to draw the fresh water through the pump and into the bowl. That will be a good thing to do once you are using the boat as intended, because the fresh water rinse at the end of a trip flushes out the little marine critters that stay in the pump, die, and then stink. So that's a good mod to make regardless, and it will also solve your most immediate issue. You would, of course, need to have a source of water to pour into the sink, but I'm assuming that's not big deal.
Peggy Hall, perhaps the world's expert on marine sanitation (no joke!), discusses this mod
here.