strongly reccomend you use an eternit waterproof board or something similar, for backing the shower walls. If you are boarding the floor use 18mm on floorboards or 25mm if straight on joists.
You should not use plywood as a substrate for wall tiling except in small areas - too much scope for movement. Then only use WPB grade.
Use plasterboard for the non-permanent wet areas. For the area around the shower either plasterboard OR aquapanel and tank the area as well.
How thick are the existing floorboards? I'm guessing probably 18mm. If you use 18 WPB ply on top plus the adhesive/tile depth (say another 12mm) you'll have an inch step up into the bathroom. It's better to replace the boards in the areas you are tiling with 25mm as suggested above (use WPB) and I'd consider using a decoupling membrane as well.
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