Re: fics - Just google up site:TF2chan.net paired with like watersports or piss and see what comes up. There's quite a bit of stuff - not all of it relevant, but there's stuff. (Sorry I don't have more specific recommendations than that - I haven't read through very many.) ...Actually, I just realized a number of hits are people saying this kink's not their thing or explicitly one of the few things they won't do (fics/art), so I guess that's our answer for why there isn't enough piss kink in this fandom for how much of a no-brainer it is. Much sadness.
In other news, I found one of the pics I saw earlier but lost track of. http://catbountry.hipstr.com/post/22804153815/what-a-lovely-tea-party
(Truthfully I'm a little "eh" on the, uh, special effects, but it's something.)