Fill in the gaps between the tiles using a rubber grout float.
Gap between shower floor and wall tile.
Similar to the gap that can exist between your wall tile and tub you may have a space between the floor and your bathtub.
Leaving a 1 16 gap between the tub and tile will prevent the tile from cracking when the tub expands.
In addition leave a 1 16 to 1 8 expansion joint between the main wall tile and shower floor tile or adjacent wall tile.
Place the 1 16 horseshoe shim on tub and measure up to the laser level.
Grout is generally the best choice for filling joints between tiles in showers or elsewhere.
Some like to use a ledger or other space saver to indicate where your missing row should be.
Grout the gaps between the wall tiles take out the spacers and prepare the sanded grout mixture.
Solution would be to partially fill the gap about a half way with grout and to finish of read seal remaining with commercial grade 100 silicone caulk.
And after installing the floor tile you can install that final or first row of wall tile.
If your gap is smaller than 1 8 inch in width use regular caulk.
But where two tiled shower walls meet or where a shower wall meets the floor called change of.
Fill gaps larger than 1 8 inch.
Hopefully these tips help you with your project.
Plus a 1 16 gap allows the caulk to properly fill the space and waterproof the transition between the tile and tub or shower.
As with any other gap near your bathtub you ll want to close the opening to prevent any possible water damage or the onset of mold and mildew.
This gap may widen over time as your home shifts.
This joint should then be filled with 100 silicone to allow for expansion and contraction as well as providing a waterproof layer.
Since you are joining two different materials together you should use caulk in the space where the floor tile and tub meet.
However due to different rate walls and floors expand grout will eventually crack again leaving you with the new hairline crack.
Or fill the gap with grout.
Soak a sponge in water and wet the tile with it.