Building a dock ramp for a dog
Your idea may work in the short term, but over time the waves will rip that apart. The other thing is that plywood in water won't last long either, even marine treated plywood is not designed to be submerged.

Here is what I did on our dock and above ground pool to allow our dog access.


Be sure to use pressure treated wood. The step allows the dog to swim up, get his front paws on for grip and then pull himself up out of the water.

It will take a little training but nothing a few treats for motivation cant fix.

Adjust the step for the size of dog you have, the smaller the higher you place the step