I'm inclined to agree with huldrejenta on the vase thing--Tom obviously suspected that she was an outsider, and therefore Significant, from the very start, and during their talk he was only getting more and more hints (Polly's favorite thing is being things, is it?) -- but the vases could have been confirmation.

I would love to feel more certain I understood the water vision, though. Understanding this might clear up the end, too. ;) Is water significant anywhere else in the book? Wasn't Polly at a swim meet, when Tom held the umbrella for Granny and Granny scared him off? (My room is dark right now, can't/don't want to check. ;)