I've been crawling through the web searching for something (affordable) I could use but I can't seem to find anything reasonable. Pretty weird as you can get a cheap $35 fish finder on Ebay. The sensor would need to be in-hull mounted meaning that the sensor would need to be able to ping through the hull (which the aforementioned fish finder does).
I guess I could buy the fish finder and a) use it (boring) or b) use its sensor (not sure how).. I don't need a huge range, something around 30 m / 100 ft would do fine. Ideas?