An easy way to reverse a motor (or motors) in a twin set-up using only one ESC (such as Electronize) is to use a polarity reversing toggle switch. I used this method in my MTB until I replaced everything with twin car ESCs. The switch is operated by a servo using a spare channel on your TX, and is just a rod through a hole drilled in the toggle with a stop each end (at the appropriate distance apart) and silicone tube buffers each side of the toggle.
Power goes in the center 2 terminals and the motor wires are arranged in an 'X' across the 4 outer terminals with the opposite pairs joined and hooked to the motor + and -.
You could make a double action set-up with 2 switches (reversing both motors to opposite directions) if required (I was only reversing one motor) To operate, you stop the motors, switch toggle to reverse and go either forward or reverse on the throttle (one motor will be working as normal, the other opposite). I only used 1 on my model and it was sufficient to give good independent maneuvering.
Check wiring polarity carefully 1 pair at a time to make sure they are switching properly before you solder the pairs. You may need to adjust to suit your desired motor direction when you move the throttle so it's best to set it up out of the boat. The unit can be made much more compact, as mine was a prototype using bits I had lying around.