Thin set or spackle to fill home before tiling
Drywall compound is the correct material to patch drywall with.  I'm curious why you aren't tiling to a fiber cement product, and suggest using a waterproofing membrane like Hydroban if you are really worried about water infiltration - https://www.google.com/search?q=hydro+ban&amp;source=univ&amp;tbm=shop&amp;tbo=u&amp;sa=X&amp;ved=0ahUKEwiLspa0_7HZAhVnp1kKHbQ9DyYQsxgIKw&amp;biw=1760&amp;bih=839 is very easy to work with.