Am I misunderstanding the specs of this solar panel or electric motor?
The answer is simple:- you chose the wrong motor. For your application you need to turn a large prop, but not very fast. So you should be looking for a geared motor or an outrunner, with a rating of at least twice the power you will put actually into it (because you want to run it at close to peak efficiency, not maximum output power). 

12V trolling motors typically draw about 500W, which would require 10 50W solar panels. Put the same power through a low Kv outrunner such as the Turnigy Aerodrive SK3 5055-280kv, and you should be good to go. The only other question is what size prop do you need to draw that power?