tiling on to plywood

[PHG Dave]

i would start a over and use
Weather & Boil Proof (WBP)

This is what most people usually mean when they say "marine ply". It is a high quality ply suitable for outdoor applications. As its name suggests it will remain waterproof, and won't blister or de-laminate even if soaked and then left to dry in the sun. Unlike marine ply it may have small voids, and is not suited to constant immersion, but is cheaper. Finish is usually nice and uniform (if a little plain) with no visible knots in the outer surfaces. then tank it with bal water proofing kit and go from there i hope this helps...

WBP plywood, even the best quality, is NOT waterproof.
Also there is too much chance of coming across substandard ply stamped as WBP grade that will delaminate if any moisture gets to it.
Plywood should not be used as wallboards to tile onto, use either a cement based tile backer board which would be water resistant, or Marmox boards which are waterproof