Here’s the full stanza from the paq'batlh:

ghIq QavwI'chaj DuQchu'

  qeylIS betleH

  chaHDaq SIStaHvIS negh 'Iw

Then Kahless's bat'leth

  Pierced the last of them,

  Showered with the soldiers' blood.

{chaHDaq SIStaHvIS negh 'Iw} “while the soldiers’ blood rained on them”

I’m not sure this supports your idea that *{SISwI’} may mean “rainwater”.  I think your first instinct was correct:  if anything, *{SISwI’} “thing that rains/showers”, e.g. a rain cloud (or maybe even a shower or sprinkler head).

I don’t remember if anyone has mentioned this, but we also have {SIS yoD} “umbrella” from TalkNow ("rain shield"??).  However, lojmIt tI'wI'nuv reported a personal discussion with MO at qepHom 2011:

 "Note: the existence of a noun *{SIS} ‘rain’ is probably limited to this

   expression (i.e. it's a fossilised remnant rather than a productive noun)."

--Voragh

On Behalf Of Felix Malmenbeck
Sent: Wednesday, March 22, 2017 7:19 AM

According to an e-mail by Roger Cheesbro:

> All correct. SISlu', altho grammaticlly correct, he didn't particularly
> like. Someone COULD use it but to me it sounds like they skipped science
> class and don't know what the subject is. You can also give it an object
> and say things like the clouds rained down cats and dogs. ...or something
> like that; you get the idea. But when Marc and I went outside and drops
> of water were falling on us, he looked up and simply said "SIS".

This would indicate that a {SISwI'} would actually be a rain cloud, rather than the rain itself, which would be {bIQ SISlu'bogh}.

However, in paq'batlh (paq'raD, Canto 13, Stanza 6), we have the sentence {chaHDaq SIStaHvIS 'Iw}, indicating that rainwater may indeed be described as {SISwI'}.