There are various factors to look at; the current flow, water condition, type of jig you used, the presentation of your jig and most important part is to be hardworking.
The deeper the better but be carefull of the seaweed and coral below.
Actually, i don't think depth was an issue. For my experience, squid normall will hide around the place where have rocks or smthing which they can hide them self. By the way, i think around Punggol jetty was nice place to get them.