Raythem
I debated the 'fishfinder' vs. 'depth no. read out only' for my Catalina 22 last winter. You can get fishfinders for relatively cheap. But, they would likely have to be mounted on a swing-arm from the cabin as mentioned. I went w/ the Raythem Bi-data 20 for $300. It has speed and depth. Very happy w/ it so far; easy to install and use, simple screen (if not fishing, really no need for a bottom read-out), mounted to cabin exterior. Regaurdless, I recommend getting one that gives speed, too.