Hi there, I've looked into that myself as well, and the answer that I've gotten back from various sources is that the Pow name being predominantly from the Edinburgh area does not have it's own Tartan pattern. Traditionally tartan patterns were for highlanders (northern Scotland). As such even the city of Edinburgh does not have it's own traditional pattern. Although in modern times many "new" tartan patterns are created for all sorts of things.
If you want to research it with a tartan maker, I recommend www.tartanstore.net
They have a database of over 13,000 patterns from old traditional ones to newer modern ones. The Pow's did however have a family crest which I have a copy of.
As for Pow having to do with water, there is a creek in Scotland near Fife called Pow Burn, named after the family. You can look it up as well on google maps.