The Ice types in the Emerald Pokedex are Snorunt, Glalie, Spheal, Sealeo, Walrein, and Regice.
Snorunt is found in Shoal Cave during low tide. Note the tide doesn't change if you have a dry battery cartridge.
Glalie must be evolved from Snorunt.
Spheal is found in Shoal Cave. A specific tide level isn't necessary.
Sealeo must be evolved from Spheal.
Walrein must be evolved from Sealeo.
Regice is found in the Island Cave by first solving the Sealed Chamber puzzle, then following the steps within the cave.
You're also able to transfer any of the other Ice types that existed up to Gen 3 into Emerald, but these are generally tedious to obtain, as they don't appear in Hoenn naturally.
All the Ice types come late and none of them are very good, so you'd probably be better off teaching a Water type Ice Beam if you want Ice moves.