Grout blades are available in specific sizes designed to correspond with your grout width. A little bit this is so you can make exactly one pass, but mostly it’s so they’re guided by the sides of the tile and you can’t really get them to dig in. If you use a blade that’s significantly thinner than your grout width, you’ll need to spend a lot of effort making sure your blade stays parallel with the tile edge. Naturally a blade wider than your grout line isn’t useful at all.
As an aside, “a bit of water going behind the ceramic” is not a failure scenario. Grout is not waterproof (tile isn’t by design but often is practically); a well designed shower should be just fine with a little water getting through the grout, all of the “keeps the water in” stuff should be behind the tile to start with. You’ll need to be careful with your grout removal not to accidentally cut through whatever’s back there (tar paper, sheet applied membrane like Schluter, liquid applied membrane like Redguard, plastic sheeting, whatever). Now, that said, lots of showers have nothing but drywall behind the tile, if that’s been wet for a while then fixing the grout isn’t really going to help, it’ll continue to mold and degrade.
edit, if it’s a floor, cracked grout is usually symptomatic of tile movement, either from a loose tile or loose subfloor – it could just be a small point where someone dropped something I guess. Fixing the grout won’t fix that either. And again, you shouldn’t really be in a scenario where enough water is getting through to be a problem.