About six months ago I used marine plywood in a baptismal for a local church. Because the plywood is treated I wouldn't recommend installing a standard lime based thinset directly over it.
If you can I would nail some wire lathe over your plywood and install over that but if you cant...
Sand your plywood down with some 60 grit sandpaper to rough it up and then install your tile over it with epoxy thinset.
It wouldnt hurt to apply a thin coating of epoxy over your plywood first with a putty knife and allow it to dry before tiling over it.
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