Since it's a tub area you will have two seperate layers..
check the lit, but you likely want to "dry in" towards the tub
and 'dry out' for the outside. So you would have tile, foam
(w/maybe 3/8 foam under the furring), and a 6-mil poly barrier,
then the masonry and the outside. The masonry wets+dries
on the weather side and the tub surround wets and dries inward. The barrier between prevents the sun or weather shifts from driving the masonry moisture into the house,
so that's a hard barrier (the 6mil poly).
A tile surround needs the special mortarboard under it,
but a fiberglass surround you can have an airgap behind, and
there's no special drying path...