Marble is porous. More of an issue with floors, likely, in areas where moisture is a common occurance. If you can find a ceramic tile that meets your aesthetic (matching patterns and colors) and non-slip reqs., that would be my suggestion for material use in this area. Happy Houzzing.

Glass is also a great non-porous alternative.

Use a proper sealer. Travertine makes such a gorgeous shower.... and timeless. I hate ceramic even though some looks pretty good, why not get the real thing? *Ask you tile man what he'd rather work with... ceramic is hell on tile saw blades.