Oil for smoke or stains, Acrylic for mold mildew, acrylic is a synthetic resin so it will not feed the organic growth, it is resistant to it.

then main main coating -> Cut in (like house painting) around edges, penetrations, hardware; Coat main surfaces.You'll probably wind up using a good acrylic (cheap and easily available), to a silicone mix or pure (costly but lasts longer). 

I was told that acrylic paint might be an option.

Latex or siliconized acrylic, both paintable, must be used when either surface is painted.

The best advice I can offer is to ask him to use an ACRYLIC sealer(not water based!)

I stayed at a fairly new Springhill Suites in Denver a few weeks ago and being in this business, I always take note of the fixtures.  The bathroom walls were done in what looked like tile, but cop a quick feel and you...

It's not a full "rebath" as the old tub was removed and they used "acrylic " walls.

Do you suggest using acrylic showers/tubs or porcelain over steel?

Like Ronald said, make sure your cap is good too, if not use Portland cement with an acryl 60 additive mixed in with it. 

I am considering acrylic shower pans or tile for the shower floors in my rental units.