I first got interested in watercolour collage when I stumbled across the work of Gerald Brommer. He's written loads of books on the subject, and although he doesn't have a big web presence I was able to find a couple of spots where his work is online. One is Fishink, where I found some examples of his work to showcase:
He has some of his work at New Masters Gallery as well. While I was vaguely familiar with collage as a modern art technique invented and popularized by famous artists Pablo Picasso and Georges Braque, it was Gerald Brommer's use of the technique in landscapes, especially his hilltop villages, that really got me going.
I've used collage with watercolour several times now, both as a primary technique and also as a way of "rescuing" a painting that didn't seem to be working (or where one area had become an irretrievable mess). It is slow but fun, and is something that I found I liked to do alongside a "pure" watercolour of the same subject. While one dried, I could work on the other. Collage does create a big mess relative to watercolours, but it is more like fun and less like work, at least to me.
Maligne Lake, 15" x 22" watercolour, torn paper, and acrylic medium
It's definitely an area for further exploration, perhaps with acrylic, perhaps with ink.
For those interested in the technique, whether with watercolour or other media, I highly recommend Brommer's book Collage Techniques: A Guide for Artists and Illustrators.