What are your sub-floor materials now? Also I am concerned with your basic level of knowledge and the complexities of building a proper tile base in the shower and waterproofing under the tiles. I hate to sound discouraging, but tiling a bathroom so the installation doesn't leak and fail subsequently is one of the most complex projects you can take on. The forums here are full of horror stories concerning people who were supposedly professional tilers who screwed up, and it's a project you will come to regret as a DIY-er if you don't have much experience with this (as I suspect is the case from your questions). I'm not trying to insult you, but I am trying to save you from having a mess on your hands. Tiling a bathroom is much more complex than tiling elsewhere in your home.

What does your expert tile installer tell you? The expert who always follows the National Tile Contractors handbook?

Not all tile needs cement board. Not all tilers use cement board. All 'wet' areas need moisture barriers. I'm more concerned with the water proofing than the cement board. And no - cement board is NOT water proofing.

Is this a DIY project? A lot more goes into tiling a shower than just laying cement board, which is not always needed. Have you researched using marble in a shower? Not always the best option. There are a lot of porcelain marble look tiles that will function much better.

Have you researched step #1. Does your floor assembly meet deflection requirements for marble as they are more stringent than tile? Does it even meet requirements for tile? Just making a point. If one thing is wrong it all could go bad. Not criticizing but your questions makes it appear you are in over your head. Showers are not for the average DIY'er IMO.

The following link will help you with the starting point I mentioned above.

https://www.johnbridge.com/vbulletin/deflecto.pl