Recess for soap and other items within showers is not a good idea as it eventually allows seepage and rot to occur, even if waterproofed. Normal expansion and contraction between different materials eventually causes cracks to the waterproofing underneath, then leaking. These recesses must be monitored and kept sealed to their internal corners.
Here is a house that is approximately 7 years old. This shows why it is very poor building practice to fit particleboard and not compressed fibro sheeting below wets areas of a building. Leaking will not cause damage to compressed fibro sheeting.
Same problem, different building.
Here is another reason why not to use particleboard as flooring, […]