Not enough minerals.
So, I just c/p'd this image. Anyways, This is simply a graph of the salinity of the ocean as the glider traveled. As you can see, since the glider was probably released near the mouth of a river, (maybe the Hudson?) initial surface water had a low salinity. However, since salt water is denser than freshwater, there is a patch of saltier water below the brackish water. Then, as the glider traveled farther out to the deep blue ocean, the water naturally got saltier, since there is less freshwater diluting the denser ocean water. Of course, water with more solutes i it is denser than pure water, so salinity increases with depth. Nonetheless, there are some areas where the water suddenly becomes slightly fresher. Maybe this is the work of rain showers? I'm not entirely sure. My brain tends to function a little slower at 4am.