What type of plywood for shower bench?
I would skip the plywood and use Kerdi board. It will cost a bit more but it's very easy to cut, very light weight so it's easy to handle, and it is completley  impervious to moisture.

You can get it 4x8 sheets and build a custom seat or one of their pre-made seats, if you can find one in a size you can work with.

Here's a video from the maker of Kerdi-Board about making a bench out of the material.