To answer the salesman who said that it wasn't necessary, tell him that you would like to perform a demonstration on HIS boat that DOES NOT have a circuit breaker installed.
Take a piece of good heavy wire, and stick it into his trolling plug socket - shorting out the + and the - wires on his trolling motor and RUN.
You will be lucky if it only just melts down the wires and does not blow up the battery and set the boat on fire.
This would be a very graphic representation as to why a breaker is necessary.
On the other hand, if you do exactly the same thing on your rig with the circuit breaker properly installed at the battery - the short at the trolling motor socke will result in a spark and a click at the breaker as the breaker trips. No fire, no melted wires, no blown up battery and no burned up boat.
Remember, it is not that unusual to have a short develop on one of the wires that extend to the motor, if a knife happens to slip and cut the insulation, or if someone steps on the wire with something sharp on the bottom of their shoe, or the motor itself fails by developing a short somewhere internally in the motor.