Shoal cave is North of Mossdeep City, walk around the left side of the island in the shallows then surf from there. If I remember right Snorunt can be found on a lower level where your find ice. At different times of the day the cave is either flooded with water or not. Different parts of the cave are accessible depending on whether it is flooded or not. If you can't find your way down to the lower level you may have to come back later.

where is shoal cave located...i need a snorunt...and I think there i can catch it to evolve it to a glalie :D
i only have sapphire and i think it exists in saphhire..lloll

You can get there during 3-9pm when it's low tide.