Don't put marble in a shower... natural stone 101. Can you go back & do it now? Sounds nice. Can we see a pic?

As far as marble counters are concerned, polished marble does not need sealing. Are your tiles honed or polished? Etching happens as does water absorption on floors with or with out sealing and sealing only minimizes stains a bit. Marble is not hard enough to be a great shower floor material other stones maybe.

Sealing does nothing but give you a skosh extra time against the red kool aid. It ain't gonna help you a bit with the real issues of marble. It won't prevent etching at all. Or chipping. Or impact stars. Or scratches. It's is basically completely wasted effort for the most part. Of course, if you want to pay a change order for a bunch of wasted effort, that is up to you. Knock yourself out. But, sealing is never part of a standard install. It is always an a la carte extra fee.

Listen to Sophie - everything she wrote is accurate in my (much more limited) experience (we have marble in several rooms, all installed by experienced, well regarded GCs or their subs).

The tile installer told us he didn't seal anything. We were just told by the GC the tile installer sealed the slate in the laundry though (note that it wasn't specified in the bid for him to do it there) but didn't seal our marble in the master bath because it is all polished. He is wrong... the floor and shower pan are honed. So basically he has no idea what he is talking about.

While I'm sure marble showers/floors is not for everyone, we had marble flooring as well as a fully marbled shower in our last house and never had a problem with staining or chipping in the 3 years we lived there.

Let me ask another question - when installing marble, is it recommended to stack pieces one on top of another or should you take care not to stack marble so it avoids scratches? We have several pictures we took during the tiling process where we saw the tile installer had stacks of tile on top of the pieces that were already in place. Before the project began I went through our subway piece by piece as we have some areas were the marble is very visible and other areas not-so-much so I separated them accordingly. I took out any pieces with scratches and chips. On our walls I see pieces with big scratches. Is stacking marble pieces common practice for a professional?

And lastly, how hard is it to bullnose? He said he couldn't do it but our last tile installer who did tile installation as a side job could and did so beautifully. Again, was he just being lazy?

All the marble I've had installed was sealed (and with the product of my choice). Whether that was priced as an extra or not, it was understood to be necessary and included. Marble is going to show some age, but a top notch modern sealer (I recommend Drytreat Stain Proof Plus, which is a piece of cake to use yourself) will inhibit staining for sure. Any chips, white impact marks, etching blends into the (typically busy) stone over time.

I used Caesartone in one application (not a look alike) and it certainly isn't bulletproof to chips and other issues ... for the pure beauty side of things, just no comparison whatsoever to real marble ... disagree with Sophie's "know what you're getting into ... and once you do, you'll likely make another choice" words of wisdom.

My fear with marble is rust bleed leaching out and generating orange spots on the stone. It's why I have avoided the use of marble in the shower where it's chronically wet.

Not sure on the stacking stuff, but marble is pretty soft ... any good fabricator/tile installer can do anything with it (e.g. bullnose) without struggle.

I'd pick up some Drytreat/Akemi/Bulletproof and do the sealing yourself ... very simple.

Momanda09, when said marble is packaged it does not have protecting sleeves in the box for each piece of marble. The tiles are stacked within the packaging. Stacking tiles around the job is common whether it be natural stone or man made.
If sealing the stone is not specified in the contract, then more than likely it will not happen on the job. No, it is not a standard for the installer to seal natural stone. Who's supplying the sealer and paying for the labor involved? It has to be specified in your contract. Make sure your GC isn't adding an additional charge for the sealing job that didn't take place, per the installer.
Honed and polished surfaces take different types of sealers. Avoid water based sealers for showers, only use solvent based sealers. Sealers do not make a product moisture or stain proof. They only slow the process of the stain, allowing for easier cleaning.

@Mike0814, rust and iron deposits are natural in all natural stone. It is more common with Asian based quarries. If your marble came from China, you have a better chance to see your marble turn red or green.

Sealing is an easy enough DIY should you want that to happen. It will have little effect on how marble wears in a bath. It's only useful on a counter where you are likely to see the occasional product stain. Even then, it does not make marble bulletproof. It only slows down the absorption of stains. It will not make it waterproof. That should be done during the construction of the shower. Marble is porous, That will not change with a sealer.

This topic has taught me how much energy I saved myself by going with porcelain tile in the shower. It looks enough like marble for my eye, but we did a lot of shopping.

Have no fear you can do it on your own. We use Mapei Porous Plus. You don't need an expert to seal it. if you have natural stone. Invest on a good stone cleaner to clean the stone every so often and seal it once you are done. Do not use any acids etc to clean the stone as they will eat away or stain it. The stone sealer will also work on your kitchen granite as well. Just apply and wipe off. Especially with marble once you see it looking dull....get your stone cleaner and do the sealer. Be careaful with leaving water or chemicals on the stone for too long as marble will stain. A sealer is just like furniture polish wipe on and wipe off.

Porous Plus is a poor choice for a dry area. It is an extremely poor choice for a wet area. DryTreat or Tenax ProSeal are the only two worth using in a wet area. But, nothing will prevent etching.

Thank you all - I will check out those products. He is going to seal our honed marble on the floors/shower pan but not the polished marble on the walls/curb. What do you rec'd cleaning the marble with before it gets sealed?

The marble should already be cleaned of grout residue. If it’s not, you have bigger issues than a waste of time applying an expensive product that will likely just cause issues. If your shower is properly waterproofed with a topical membrane, then you’ve done all you can. If it wasn’t, then nothing will ever be enough.