We have seen a similar issue in the past. The problem occurred when the porcelain tile on the floor did not have 100% contact with the tile adhesive, this caused water to seep down though the grout join and hold in pockets under the tile in the areas where there was no direct contact. The water being trapped in the pockets under the tile does not evaporate and can leave the tiles looking stained. If this is the case with your floor tiles the bottom row of carrara marble tiles are simply acting like a sponge drawing water up. To fix the problem we would suggest that the bottom row of carrara marble and the porcelain tiles on the shower floor need to be removed and relayed ensuring full adhesive coverage. Unfortunately this may compromise your under tile heating and waterproof membrane. Proceed with caution.

Why would a tile shower floor be a problem? If the floor is properly waterproofed before installation, it should be fine. Most shower floors are, after all, tiled.

how disappointing ...has the tiler offered any solutions? in the meantime a simple shower curtain could be worth considering.. extra long white shower curtain (213cm hangs higher and looks better) about $25 from Spotlight and "extenda rail" adjustable tension rod available for about $20 at Bunnings...easy DIY quick fix without making any holes in the tiles and could be worth trying until the perfect solution is found

Having experienced a leaking shower, a step down sounds like a welcome added precaution, which seems rarely done. Supposedly if there is no hob or atepdown there is an angle, and obviously the whole thing is supposed to slope towards the drain anyway, but BeachBathrooms' earlier comment about capillary action rings true. It became obvious very quickly in our other shower that water was creeping across the grout outside the shower so whatever was meant to be done either wasn't or wasn't correctly done. Then there's the modern hotel bathrooms I've experienced where the water simply splashes out too easily. All in the name of style.