Replacing tile without backing

Hello!
I have an apprx 15 yr old pool that has a row of decorative tiles along the inner edge/where the water line is.
While I was pressure washing the pool walk, a few tiles popped off and, surprise, there is just soil behind where the tiles were. I have no pool knowledge, but a video I watched showed tile being replaced onto a concrete base, so I'm assuming the folks who put this pool in did a shoddy job of it.
Anyone have a suggestion on how to replace the tiles without a solid backing?
I can't do a thorough/proper job of repairing it right now, just looking to replace the tiles so the soil doesn't start washing into the pool.
It seems there is leak somewhere, too, below the tile in the actual pool wall, so if you can also suggest a product to fill cracks on wet surface that'd be awesome.
Thanks