Adelaide's Outer Harbor is the gateway for ALL vessels arriving and departing greater Port Adelaide; they must traverse the breakwater channel, whether to berth at Outer Harbor itself or up river at Osborne or the inner Port Adelaide.
This location is served by several recently built viewing platforms atop the breakwater behind which are lawns and shelters. There is easy access by road and rail [the station is five minutes' walk away]. There are no services so bring your own drinks, snacks etc.
If you are a visitor, access the Flinders' Ports website to ascertain all ship movements at this and other South Australian ports. South Australia has become extremely popular for cruise ship arrivals mostly between late October to Late March each year.
Photography is unimpeded; a modest tele-lens will get you everything.Best pics are taken in the fore-noon with the sun behind you but it is also great for departing well-lit vessels at night.