That's about it Colin, the lower hull is steel and the upper superstructure is aluminium. The joint between the two is, as you say designed to insulate the two and prevent galvanic action although it is nothing more sophisticated than a jubber joint and the rivets are fitted with insulating sleeves.
There has been significant deterioration of this jointn over the years and most dry docks include a section of it being replaced.
As for cracks, there have been some cracking issues in the past, particularly around the joint as the two differrent metals not only have quite differrent electrolytic characteristics but they also differ physically. One of the problems with aluminium is that it work hardens so when subjected to stress reversals as in a ships structure the metal can become considerabkly harder than the steel and therefore less flexible. The continued flexing of the hull has then occassionally caused cracks to form.
Interestingly enough if you stand at the aft end of a large enough ship in a heavy sea you can see the flexing of the hull quite clearly. I remember one ship particularly where you could line up a row of container supports and watch the waves actually progress through the length of the hull.