Your best bet is to use the most easily attainable steel component for the area you are building you dock in, and then make sure it is bonded thoroughly.  Install cathodic protection to the steel and not worry about it again.

Is cathodic protection of rebars coomon and popular enough
for new constructions? What about capital and operating costs?
How many projects have had CP system of rebars installed?

You should use black or hot dipped galvanized bars, the latter is better. I never use epoxy coatings in a marine environment because you can't economically avoid holidays in the coating. In some environments, a holiday can promote very high rates of corrosion in localized sections of some bars.

Cathodic protection is very good, either active or passive. Hot dipped galvanizing is a form of passive cathodic protection and it also acts as a very tough barrier coating. I've even designed galvanized bar systems with additional sacrificial zinc manganese anodes to extent the onset of first corrosion.

Regards,