Fixing grout issue on decorative swim-out tile

When our pool was built last spring, I didn't notice that some of the decorative tile in our swim-out either didn't have enough grout placed between the tiles when they were installed, or what grout was placed there didn't stick / got brushed away, leaving a significant gap between the 1" tiles for about 12-18" of my swimout.

I'm going to contact our pool builder this week, but I wanted to know what's the best course of action to take to get this repaired. I'd rather not have the pool partially drained just to add some grout to these tiles, and I think I've heard that there is white epoxy that can be used for underwater repairs.

Is using a whitish-colored, underwater epoxy the best material to use to repair missing tile grout? I'm not that picky that if the whitish-epoxy doesn't exactly match the color of the tile grout that's gone missing (which itself was a rather drab color of Portland cement, I suspect), I wouldn't be too troubled by it.

Thoughts?

I bought some white epoxy at our local Pinch-a-Penny store, to use as a filler between the tiles that have no grout between them. Any concerns about using the epoxy as a grout replacement (other than the epoxy might not match exactly the grout color -- which doesn't concern me)?