Replacing Tiles

A number of tiles (a dozen or two) have become dislodged from the edge of my gunite pool (I have all of them). Last year, I found a grout- or mortar-like product that I used to replace some similarly dislodged tiles (this year's are not the ones from last year). The product I used said that it could be used to repair pool or goldfish-pond time without draining the water. Although my tile wasn't below the waterline, it's just above it. I'm trying to find the same product this year but neither my local Leslie's nor my local Home Depot seems to have it (I think that I got it at HD last year).

Is anyone familiar with a such a product? Can you point me in the right direction?

Alternatively, is there some other approach that I should consider?