It is impossible to be certain from just a picture, but I would venture a guess that they are FW given the types of blemishes and the fact that you are asking the question.
The pearl on the far right of the top picture has a blemish that is normal for FW, but that isn't conclusive by any means.
Price would be more indicative. The wholesale price for a strand of round South Sea pearls from 10 to 11.5mm with moderate to light blemishing and good luster would be approximately $3k-$5k. If you paid significantly less than this (say $1k or less for the strand), then it is highly doubtful that the strand is a South Sea strand.
Tell us more about the strand. Where you purchased it, amount paid, etc... Also, are all the pearls round? Some appear to be out of round, which would be more typical for FW, but it is hard to tell for sure from the picture.
I'm assuming all along that the strand your asking about is the round, smaller one with the clasp right?
Ok. I guess we are posting at the same time here. I assumed the bottom strand is what you were asking about, since you posted a photo of it by itself. But, if it is the top strand, here is a picture of a similar strand which is a South Sea. Again, based upon just a picture...
I know the upper strand is nice and I managed to capture a bit of a rainbow which is hard to do on camera. It's freshwater for sure.
I bought the round ones second hand for just $100 dollars sold to me as South sea, but I figured that for the size if they turned out to be freshwater pearls that they would still be a fare price. I'm not really sure what they are yet but since I consider you two to really know your pearls that they are probably freshwater.