Southampton Maritme is a busy freightliner terminal and depot/stabling point.
To get there from the M27, come off at junction 3 and follow the M271 signed Southampton. When you get to the roundabout take the first exit (left) and follow this road staying in the left hand lane. Be aware of the speed camera! it takes pictures as you approach it so dont get caught out.
This left lane will take you to another roundabout, there will be a fast food outlet on your left. At this roundabout you need to go right towards DOCK GATE 20. once you pass under the flyover you will see a road dead ahead which is a road bridge. Do not go over the bridge as it leads to a security barrier for access to the docks. You need to park your vehicle prior to the bridge, either on the left or down the side road.

Now walk up the bridge, be aware because cars and lorries tend to travel quite fast over it. The stabling point and depot are on the right hand side of the bridge and the container terminal on the left. You get a good view of both. Make sure you take your bino's as there are normally a couple of 66's further down the line..
The main Weymouth Waterloo also runs underneath..

Note Its easier to park at weekends as its an industrial area.. I tend to go sunday mornings...