Trough or Dish (noun)
A long depression in the water that is visibly lower — sometimes significantly so — than the surrounding water.
Some local boatsmen reported to the Old Sow Whirlpool Survivors Association about suddenly realizing that they were within a trough so deep that they could not see the water's normal surface, perhaps 250 feet / 84 metres wide, and which was over a mile / 1.6 kilometres in length, running from just north of Dog Island, Eastport, to near Clam Cove, Deer Island.