Floor tiles - prevent water seeping under ground?

Depending upon the type of shower you are intending on installing?
If you are thinking of a walk in shower you have two options......one being the installation of a shower tray. The shower tray would be put in place of your desired location of your shower. It would be securely sat in position on a screed and a good reliable thick silicone application to the 3 outside faces of the tray, ( all sides that sit up against and wall.
Once in position secure against all wall you would again silicon all the faces of the tray in connection with all wall faces.
The tiler would complete his work so the the bottom title rests on the secure in position and twice sealed tray ....leaving one last finally bead of silicone to the finished installation between tray and tiles completely eliminating any possibility of water making it's way through 3 properly applied silicone sealings

Option 2 would be the installation of a wet room so the water from the shower shoots directly on to your floor tiles with a rubber membrane in place under the floor tiles onto a strong steady piece of plywood designed with a carefully constructed fall allowing the water to safely disperse back the the drainage position

Again on sight visual inspections would be a standard requirement so we can plan our projects to the smallest details

You would use crack mat it’s a protective waterproof matting that stops movement in the floor but also a waterproof protection it would be lay on top of the screed before tiles are put down

In any wet room sealing is crucial so providing all areas are sealed you won’t have issues water causing problems.
If it’s piece of mind that’s more important you need a dpm, screed and water resistant glue for the flooring as well as making all other areas sealed correctly.

Use a waterproofing system in the shower base and on all walls in the shower area prior to tiling

Hi,

Please use below. The best stuff.

Or tanking Kit.


Impey WaterGuard Wetroom Floor Membrane Kit – 10M2

You can use many products from latex to mapelastic to a proper hydroisolation sika product a+b components sikalastic /mapegum

If I understand you right.
a/ This sounds like you wish to have no shower tray, then it would be called a wet room. There are quite a few solutions for this online. However, as you rightly mentioned, do not tile straight on top of the screed in a wet area. Proper drainage would have to be applied to that area.
b/ If your talking about floor tiles outside of the shower cubicle/tray, you can use a Ditra Mat. (a very poplar and professional solution)

Mastic seal to make it watertight