Average offshore oceanic salinities are what dattack said. Coastal waters can be influenced by anything from rain, to whatever watershed is seeping in there at the time, and if I remember, a whole lot of Florida simply pumps it's wastewater into the channels and inlets around there. So if all of Miami flushed their toliets, you might have fresher water at your inlet<G>. Tides are wonderfully strange things. On a high tide with no wind or large wave action to provide the mixing, at a freshwater inlet you'll find that the salt water enters -under- the fresh water because it is more dense, creating a salt wedge along the bottom that can travel pretty far up the freshwater river without changing the salinity of the surface waters. If you sample the bottom of the inlet you will find higher salinities than if you just sampled off the surface water.
For fish only systems I keep my salinities at 22ppt (SG 1.016 @ 76oF) simply because having lower salinities does reduce the amount of parasties and disease. Medically I've had the salinities as low as 15ppt (SG ~1.010 @ 76oF). If you plan to have inverts your salinities should be higher, probably around 30ppt.
There really isn't any set rule as to what the salinity ought to be in a marine tank. Most books simply recommened average oceanic conditions because, well, that's an average oceanic condition. Average doesn't mean that local conditions will meet these values at all times. What you need to do is keep your tank stable at whatever conditions you choose as appropriate for your fish. Personally, I would match my tank to the values of the LFS where you plan to purchase the majority of your fish from because that would make acclimation easier on your fish.
~~Colesea
