Wet rooms need to be thought about before you go ahead and start.
Replacing the boards with ply is certainly the first step as this will help stabalise the floor. Meanwhile i'd recommend going on youtube and looking at Schluter Ditra Drain, or Kerdi Line Drain, these demonstrate pre formed showerbases and tanking systems that are fairly easy to install, and above all work.
Get an experienced tiler to go in FIRST and discuss what you want, getting a chippy or a plumber in first without consulting the tiler will lead to lots of hassle.
Good luck.