When traveling around in a boat hold
right before you hit a zombie. You'll probably have to go out of your way for most of these kills.
There is a group that spawns near the workshop in Marik's Marina that your visit early on. You can get 10 or more kills here by grabbing the boat and traveling along the shore line to the north of the marina. Once you've cleared them out you can return near where you got into the boat and there is a cave across the water. Enter the cave and then exit to respawn the zombies. Your boat will be to your left when you exit and you can simply travel in a straight line from there, rinse and repeat.