A couple quick points about anything electrical.
Its a 20% increase between a 50A and 60A breaker. This breaker likely won't trip from normal use and is there in the event of a dead short.
The breaker rating above also affects your wire selection. If you had a 60A breaker and used (for example) 20AWG wire and had a dead short, your wire would become a lightbulb filament, melt the insulation and burn everything it touches all before the breaker popped. Bad!
With respect to sizing the wire, always go with the larger size. The cost is usually minimal to upsize. Use Ancor Marine Products ring terminals or lugs for connections. Anything larger than 10AWG need a lug instead of a ring terminal. Use shrink tubing to cover the ends and you should be good.
As for mounting the breaker and wiring, you need to keep them as far away from water as possible. If you can use wingnuts on the breaker you might be able to make it easily removable. Honestly though, the option below from Minn Kota would better suit you given you are in an inflatable. No extra wiring or installing. Just drop your battering into the box and you are up and running. It even provides auxiliary power ports and a 10A breaker.
http://www.minnkotamotors.com/Products/BATTERY-POWER-CENTER/I would rate the above option as a better solution for an inflatable. My 2 cents, but at least your gonna get out on the water and thats what its all about!