As others have said, most stuff where unloading would occur is located on the actual wharves and visibility of them is tough because the ships block the view from the west, the buildings and large Port property block the views from the east, south, and north. However, there are some other options.....
At the southern end of the Washburn tunnel there is a small park actually on top of the tunnel.....on Google its labelled "Santa Anna Capture Site". Coming up from the south there is a small road that goes between a guard shack for the tunnel to the north and a large petrochemical plant on the east. Take it (no permission necessary) and it drives a couple hundred yards to the ship channel, over the tunnel and there is a little park sandwiched in there. It's on a nice grass hill and you can do whatever you want there.....big ships go by all the time.

Some even park right at the Petrochem plant there and unload, but I bet thats a timing thing.
See it.
Another place is off of Brady's Landing a large restaurant on the ship channel. They are usually only open in the evening anyway so that spot would probably work, especially if you stayed to the west of the business since its a large spit of land out there. Here you probably won't get any unloading pictures, but you will get transit pictures.
See it!
Hope that helps!