Hi Shearwater,
I see that this post may be too late for your project, but may be helpful in the future.
Bimetallic corrosion between two different metals, in an atmosphere rich in Chlorine (sea side), aluminium will deteriorate faster is a given, but, I have seen superstructures built of Aluminium on steel hulls. This practice is still followed on ships and instead of bolts the Al is generally riveted, ensuring the gaps left between the Ali and steel holes that can accumulate water are almost nil.
Another point to note is use of Gasket (3-4 mm thick Rubber/Nitrile) between the Ali and the Steel surfaces. The port hole may have to be tightened with Steel fasteners at selected points (which could be 4), so that maximum rivetting strength can be achieved. Once all other rivets are in place, the fasteners can then be replaced with rivets. Needless to mention, protective coating of paint will result in delaying the corrossion process.
Hope this helps
Rajesh