Does nuclear need fresh water? I'd imagine salt water would wreck all the equipment, and you probably don't want to use up all the fresh water you generate...
The water is just there for thermal mass to conduct waste heat away from the condenser. The water that gets heated by the reactor core and turns the turbines is isolated and highly purified, since many of the internals can only be accessed during refuels. Some designs go further and separate the reactor coolant system from the steam generator to prevent contamination, but AFAIK there isn't a single commercial design that allows outside water to feed into the reactor cooling or power generation systems. The parts of the condenser that touch the external body of water are basically just blocks of metal that have to be scrubbed on the outside from time to time.
The turbines do but you can recycle much if not nearly all of that water by cooling the steam and condensing it, which you could use sea water to heat exchange to for it.
Not really. Pure heavy water along with pure uranium and pure CANDU reactors are imported components. The quality of cooling water and feed water doesn't matter much.
Remember, nuclear power plant is merely a very large heat engine. The main problem is terrorism there.