Danoff moved closer. “It took off,” he said. Very fast, about 20 knots. The periscope sank beneath the waves, and whatever was below the surface blew its ballast, sending aloft a mighty hiccup of bubbles. After five minutes the sea lay once again undisturbed.
A sub makes perfect sense in the gulf, since the boundary between oil plumes and water shows up clearly on sonar (during WW2, dumping a cloud of oil into the water was a common sub trick to evade surface sonar). So I am going to presume that the sub is there to map the extent of the oil spillage and since they are keeping the results secret, the results are not good.