How about a 12'or 14' hard-sided boat that you can fold up to 4" flat and put on top of your car for transport. Take a look at one at www.porta-bote.com The 12 footer only weighs about 70 pounds and will take up to a 6 hp four stroke motor that'll move it along about 18 mph. There's a forum of porta-bote owners at
http://groups.yahoo.com/group/PortaBote and they discuss how they've modified their "botes" to include mounts for fishfinders, rod holders, trolling motors, etc.