Wayne:
I work in equipment design (mainly boiler feedwater components) and ocasionally see customer specifications requiring sacrificial andode to protect other (vital) components.
As you have noticed, salt water and aluminum do not mix well. Aluminum has a protective oxide layer that generally protects the metal from further corrosion. Chlorine ions (Cl-) from salt water (NaCl) can penetrate this protective oxide layer and promote corrosion of the underlying metal. (I read somewhere that San Fransisco doesn't use aluminum light poles for this reason).
It seams like you should use a different material (Stainless Steel if that is working in other installations.) I would also caution that stainless steel is susceptable to corrosion from chlorine as well.
I basically design equipment in contact with water or steam with a low oxygen presence and normally don't have much dealing with corrosion design. Material selection is an important part of any piece of equipment, though, and should be taken care of in the design phase. Field modification is often done without complete knowlege of the design (i.e. why certain things were specified the way they were)and leads to problems.
The most recognizable instance of galvanic corrosion is in a battery. There are other instances where galvanic corrosion takes place that are both good (on purpose) and bad. Using a sacrificial anode to protect a vital component is an example of a purposeful use of galvanic corrosion. I'm sure you can think of plenty bad cases.
Hope this helps!
jproj