The foxes know the penguins are there now and they’re perfectly capable of swimming across a ten metre channel for some food. Also, dredging would need to be repeated every ten years minimum. The dogs may be cheaper as well. Dog food is cheap.
How is dredging a channel an easier, cheaper or more intuitive way than a dog to keep foxes away?
Sending your dog out there occasionally seems monumentally cheaper and easier than dredging