Hi,
there's a push off button in the upper left of the screen. But you might have to change your view direction to the spot where you want to push off. Use the view arrows for the general location and direction and then (if required) pan/drag from the screen center to adjust the direction.
Press the "push off" button repeatedly if you need a bit more "power".
Conditions for "push off" to work:
#1: Close enough to the canal bank / mooring. (Like being able to push off with your feet without falling in.)
#2. Canal bank roughly in the view center.
The game currently doesn't include a method which would be comparable to pushing off with a pole. So you can't currently push off when being too far from the canal bank.
I hope that helps,
Michael