Soundproof/Waterproof Bathroom Stud Wall

I’m not sure if this is the best forum on the site, though the question relates to waterproofing…

I want to have a bathroom stud wall soundproofed (to prevent sound penetration from shower through stud wall), and also have the same wall waterproofed against water falling on it from both an overhead and a wall mounted shower, I was thinking of doing the following:

a) Insert Rockwool 50mm acoustic slab within stud

b) Screw 12.5mm Gyproc Soundbloc plasterboard onto wall

c) Screw 9.5mm Gyproc standard plasterboard on top

d) Adhere Homelux wall matting onto this, with Homelux tape for joints

e) Apply tiles

The two different thicknesses of plasterboard helps break up the sound wave.

I was speaking with a plumber, though we hadn’t been talking about the acoustic reduction part, just the waterproofing.

With regards to the waterproofing, he said he screws 0.5 inch Chinese plywood directly onto the stud. Onto this he paints some sort of tanking product, I’m not entirely sure what this is but could find out. Then tiles are adhered directly onto this.

Which method do you think is better with regards to the waterproofing?