which is the darker of the blue....
Florida or south sea...or is there a darker blue of the two available

Florida Blue is a rich "blue" blue, somewhat cobalty in sense and quite deep. South Sea Blue is much lighter and somewhat turquoise-ish.

I am using South Sea Blue right now and it has a definite tendency towards a greenish turquoise. A nice colour but not a dark blue.

The only thing darker would be Waterman Blue-Black, but then you're adding.... black.

Check out in the mixing area in Inky Thoughts. I recall some folks add a drop or so of Waterman Havana to Fla Blue and even BB to make richer.

I love Waterman inks and enjoy mixing the colors. Turq with a touch of black makes a nice dark green, or green/black. My favorite is green with Fla Blue 1:1 or more Blue. A nice dark teal.

As others have said, the answer to your first question is "Florida Blue". But, with reference to your subject line, if you're looking for a dark blue, then depending on what you mean by "dark" I would be surprised if Florida Blue is what you're looking for. It's much the same as lots of other mainstream, low saturation, inoffensive blues (Sheaffer, Parker, Conway Stewart, etc.) - a rather dull, slightly deep blue with a touch of purple in its make-up which looks insipid unless used in a pen with a fairly wet nib (yes, yes, this is all a matter of taste, and I'm just stating mine...). If, for some reason, you must use Waterman's ink, you can make this look darker and richer by adding some of Waterman's marvelous reddish brown ink, Havana (too bad they don't make a blue ink that's as good as this); improves it out of all recognition. (Waterman's makes a third blue ink, Blue-Black, but despite what its name might suggest, it's if anything paler than Florida Blue, a fairly gentle teal which starts fading the moment it dries; it too can be rescued with Havana, but you might as well start off with Florida Blue.)

I'm not sure what your second question means. If you're asking whether some other company makes a blue ink that's darker than Waterman's blues, the answer is a definite yes, several of them (some are even free of purple). For a dark, true blue, I don't think you can beat Private Reserve's D.C. Electric Blue (but don't ask me what that name has to do with the colour...). If you're wary of saturated ink, dilute it up to c. 3:1 ink: water.

Simon