Maybe you can strayline at any depth but I think generally in depths less than 30m. Particularly useful in shallows with current as it gets your bait away from the boat where the shy (larger) snapper will hang back. It's the only way I'll fish in the Tauranga harbour where I fish in 4 to 6m of water. Key is having a ball (or similar type) sinker on your main line (so line runs freely through it)and this is either tied directly to a hook or to a swivel which then has anything from half a metre to 10m trace and then the hook. I use about 1.5m trace. Use as light a sinker as possible to get to the bottom. I often have to change a couple of times as the tidal current picks up. Key is that fish should be able to take bait with little resistance - many set overhead reels in free spool with just the clicker for resistance. Straylininig spinning type reel also very effective.