You could just leave the spent primer in. But there might be weight variances in the primer cup and hammer on all those cases too.
So some guys pop out all the primers, then take one used primer cup (remove the hammer), then use that same primer cup and press it in to each case you measure then pop it out again.
21st Century makes a metal plug with an o-ring on it that you can snug into each case.
Personally I think the weight difference in primer cups is meaningless, but some guys think it matters. Most reloaders don't have a lab balance precise and accurate enough to tell the difference between primer cups anyways.
Some guys say it's not about weight but want to decap before sorting for other reasons so might as well use the same spent primer cup over and over, it's a cheap option.
See what I mean about making it voodoo? There are at least three options just on keeping the water in
I usually use the plug because I pop primers out before cleaning when I clean cases. If I'm water sorting them then there are other prep steps going on also and they get a good bath at that point.