Always Marine ply for bathrooms. You will regret not having paid the extra for it when you have to replace it a few years down the line after a small leak
As Galoka says, WBP will probably be ok, just seal the cut ends with a good poly varnish to avoid any deterioration
Marine ply is pricier but for structural use and water resistance over time, it would be the best belt and braces approach IMO.
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