Ok there are several things that you should have done when fitting marine ply in a wet area. Firstly, once the ply has been fitted, you should still seal it with an additive to prevent the ply drying out the adhesive so a PVA or SBR should be applied with a brush or sponge directly to the surface, this admix should have been watered down to 5 parts water to one part additive. If you have done that then you must be sure that you have used the right adhesive for the tiles, I.E, if you are using tiles larger than 300mm by 300mm you cannot use ready mixed adhesive as the surface area will be too large to hold the weight.
Finally, it is important to remember that when you grout your shower, it actually is advisible to not use the shower for 2 weeks.
if you have followed all of this then either the ply is too thin or not fixed securly enough. hope this helps.