I put mine just infront of the fish finder on the side of the console. I've also seen what Tee mentioned, behind the drivers seat on the gunwale. What I do like is the new Humminbirds with internal GPS antennas! I got my 798cSI before they came out with the internal, would love to not have to mount or drill holes!
You can mount them below deck too. Just make sure there isnt any metal in the way. The puck will get signals through fiberglass with no degredation of performance.
Be careful about mounting them below deck. They are supposed to have an unobstructed view of the sky. Mine was mounted below the back deck and didn't have any problems for over a year. I started having problems losing signal at the Stren championship last year and when I went to the Lowrance trailer they told me that as the pucks age they lose strength and will have a harder time getting signal.
I had the one for my LMS 332c mounted under deck on my 882 for years. Not only was it under deck it was mounted on velcro the whole time too. I never had a single issue with it.
On my Z7 i mounted it near the console. It works just dandy where i have it and where it is it does not get in the way of anything. It also made the cables really easy to run. Here is a picture of what i did.