In Java version they're literally everywhere, in any body of natural water, but in this version I can't find a single one except in Creative using spawn egg.
UPDATE
Found this bug in MCPE bug tracker, unfortunately the mod marked it as 'resolved' with 'work as intended' result, as they say Squid will only spawn in Ocean, though I've been venturing the ocean with boat for a day a night and can't find a single squid.
Best Answer
The spawn limit for water mobs is 4, so if there are 4 or more squids alive in a body of water, no more will spawn. Sometimes, there are more than 4 squids and that is because the game checks if the limit is reached first, and then spawns a large amount of them at the same time. So if there are any squids within a 256 x 256 area around you, then that is why they won't spawn; there are too many on the map.
In the Pocket Edition, there are no squids. In order to obtain ink sacs, you will have to look for them in chests in villages.