If it isn't the actual shower you're talking about, a normal sanded grout (or unsanded for thinner grout lines) should do the job just fine. Then you can seal it with a silicone tile and grout sealer. Or you can leave it be - many people leave it unsealed with fine results (and avoid quite a bit of extra work).
If you really-really wanted it waterproof, as in the shower for example, then you can go for epoxy grout which is more difficult to work with and much more expensive.