You can use a masonry bit in a drill... or, my personal fave, a hammer! (Then again, you might not be going for the "pottery shards" look, rather an actual "fishy house, so the drill method would probably work better for you...)
If you have a dremel tool, that's the best though, because you can cut a big hole in there in whatever shape you want... make a little fishy-Taj Mahal, even...