A "zinc" is a type of sacrificial anode used on a vessel.
Salt water has high conductivity and as you will notice with some of our dive gear metal tends to corrode quickly. There are a few ways to slow this process:
1) is using non-ferrus metals, these are often used for the running gear (props, shafts, bearings / also some of our dive gear like most regulators)
2) Steel or iron vessels use a softer metal attached to the hull, this softer metal will be the point of discharge and will be the first place to corrode. For steel vessels zinc anodes are used and this is where the term "zinc comes from.
3) Aluminum vessels use a softer grade of Aluminum alloy for protection but the units are still refered to as "zincs" most of the time. These are used on aluminum accessories like outboard motors as well.
4) immpressed current systems (these use a reverse flow of current to balance the draw of the surrounding sea water to slow corrosion) are also used to prevent corrosion but are not as popular on vessels, they are used fairly often on port structures (especially military)
If you get into this line of work you will see many examples of sacrificial materials, although often they are not intended to be used as such. When dis-similar metals are used in fabrication (like mixing stainless steel and aluminium) the weaker of the two is going to deteriorate at a much faster rate and this is not always calculated into design concepts.
As far as the offers for work go, if this is something your interested in you may want to investigate attending a commercial dive training center and gaining an overview of the equipment used and the regulations governing commercial diving.
As we often say out here "it aint rocket surgery" common sense will go along way, if your interested in working with a company go out on a job with them as an observer and get a feel for the operation first. Make sure the first concern they have is for the divers safety, make sure the equipment used is safe, redundant systems and well maintained, make sure they are willing to take the time to explain and train you on the hazards of the job and environment.
Then you can make the decision whether or not it is worth it to you to accept the risks, the loss of your free time, and the pay offered.
Some of the risks around a marina were already covered pretty well, some other things to consider is vessel traffic in the area, tagging out of the vessel your working on so no systems are engaged with the diver on the bottom (intakes, propulsions systems, etc.) , additional hazards that are more annoying than life threatening include outer ear infections, rashes and scrapes from the growth your removing getting inside every orifice, (make sure your employer has the insurance to cover you in the event of an injury...
