What's with the water filter on antisemitic screenshots? Does it have some special significance or history? I tried googling it (and maybe it's bc I'm bad at search terms) but I couldn't find any info.

Ah, that was something started by one of the jumblr people. It's to show that it's a screenshot as opposed to part of the post (since sometimes it's hard to tell when a screenshot ends and the actual post begins, depending on the color schemes used). It also has the added benefit of banishing Bad Comments to the Shame Lagoon.