Hmmm, this seems weird…
Go to this Flickr page: /photos/tags/water/ and this Flickr page: /creativecommons/by-nc-sa-2.0/tags/water/
The first one, which is all photos tagged with ‘water’ regardless of license, has an RSS feed you can subscribe to. The second one, which has photos tagged with ‘water’ made available under the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 license, does not…
Seems to me that you would want an RSS feed of by-nc-sa license photos, eh?
(Update: I may or may not have heard that RSS feeds for the CC stuff should happen in the future. Yay!)