If the tiles are loose, you need to remove them and reattach. Just repairing the grout won't help - even though the tiles aren't submerged, water (splashing, rain, etc) will still get behind them and there will be a cascading effect of popping tiles. Better to fix it sooner rather than later...

I just went through this a few weeks ago, I had about six tiles that loosened up near the skimmer. I ended up using a 50/50 mix of limestone and portland cement, mixed with water. I used that mixture as well to grout with - I didn't really car how it looked since I plan on retiling next season.

So far so good, the tiles are firmly attached and have been partially submerged in water since I finished the job. No cracking grout, etc.