I don't worry about a breaker on my transom mounted TM that I use on my inflatables. If it were on my big boat (where the batteries are in a compartment) I would.
The battery is going to be sitting right beside you and there really shouldn't be an issue.
If you really want to put one in and save money just buy yourself one of these from Napa as they are under $10.
Product Description
Line code BKP
Part number 7823001
Description Circuit Breaker
Attribute Amp:50 Amp,Terminal Style:Threaded Stud,Product Features:(2) 10-32 Threaded Studs & Crosswise Bracket, Type I, Auto Resets After Tripping,Voltage:12 Volt DC,Mounting Bracket:Yes