I recently came across a really neat waterproofing underlayment product, Stratoflex. It is a modified bitumen (asphalt) that resembles the rubbery roofing membranes used on commercial buildings. This material can be used as an underlayment for tile and would act to seal any water out of the concrete. I have no personal experience with this, but the concept is sound and might be a good solution. Try this link.
Sikaflex is used in sealing everything from windshields to building panels. The xylene in Websheen is only a solvent (evaporated) and the acrylic sealer solids remain and are supposed to seal the concrete. You could use additional coats of a similar product.
Another alternative would be to install a roof over the deck. That is the configuration I am accustomed to seeing when there is finished space below.