Having no short-circuit protection in a boat (especially a fiberglass one) is asking for trouble due to how fast a boat can burn. With aluminum boats being conductive the boat itself can become the short circuit .Some trolling motor makes come equipped with a circuit breaker mounted in the foot pedal sized to protect the trolling motor . This will only protect the circuit from the breaker down stream to the trolling motor leaving from the bow plug wiring back to the batteries unprotected. A circuit breaker should be installed to protect the wiring running up to the trolling motor plug based on the wire size . #10 wire = 30 amp , #8 = 40amp , #6 = 50 amp .
A trolling motor battery is capable of incredible amperage in a short circuit condition determined by the resistance of the short .
Lets assume that we're running a 24 volt battery system that draws 24 amps at full load (just to make the math easier)
Using "Ohms law" we can calculate circuit resistance at full load /24 amps 24volts divided by 24 amps = 1 ohm resistance
Now lets say your trolling motor wiring has damaged insulation resulting in a direct positive to negative short circuit .
Now the only limiting the short circuit current is the resisitance of the wire ( lets assume .01 ohms)
24 volts divided by .01 ohms = 2400 amps ! I don't think the batteries could put out that much current but bad thigs are going to occur with no protection such as wiring catching fire,boat catching fire , batteries exploding . West marine has a great assortment of marine circuit breakers (that just where I buy my stuff) and I would recommend adding one if you dont already have one in your boat .