Mod Podge

I believe Michael's craft stores carry it.

Walmart, too, IIRC. 

Thanks, found it on the Walmart website. (In store only)

I found it in a number of stores - but I found a few different variants of it (Matte, Gloss, semi-gloss).  Which is recommended for water?  Are some recommended more for rapidly running water and others for still water?  

Thanks!

I buy mine at Michael's

For water, I use the gloss Podge. It takes days to dry.

Once it does dry you may want to seal it with a coat of waterbased poly (cause it can get mushy in the humidity)  Plus its easier to dust off and keep that shiny watery effect

MICHAELS. thee are always 40% off coupons available you will not get the depth effect as with other water products but it is a good effect nonetheless

For depth, I use Mod Podge over window glass (painted on the bottom). Much cheaper and more available than the wavy shower-door glass that many prefer.  Plus, you get to scuplt your own wave patterns. The only downside is that it is a bit harder to keep clean than raw glass.