There is a guy that free dives out of Rye and picks up scallops in the bay at about 8M, and dives for crays on the ocean side. He also spears the big squid when they come in to spawn at the end of winter.
I'd love to free dive, but frank it scares the sh*t out of me. I can't hold my breath that well.
Squidder is spot on. I've collected them from 6 - 15M. The ideal depth seems to be between 8 and 13ish. The bigger ones tend to be in deeper water in my experience. If you dive down and you see them, you'll get heaps. If you dive down and can't see any, you'll struggle.
I've seen them collected straight out of Anthony's nose boat ramp (Dromana Power Boat Ramp) all the way down to Blairgowrie.
Tides aren't really a problem around there, but the boat traffic is.
If you are diving anywhere near the heads, you would have to time your paddle to coincide with slack water, otherwise you would be on a drift dive.