I don't think there is any allusion to shoals of fish here.
In the first stanza, there has been a storm at sea, with four survivors and forty lost, "gone down together into the boiling sand."
The "four" and "forty" are presumably sailors, or ships' passengers, or even the number of ships sunk by the storm or saved from it.
A "shoal" is an area of shallow water. Because of the shallow and uneven depths of water, the currents in such an area are often fast, hard to predict, (the direction changes from point to point as the depth varies) and dangerous.
A ship which gets into difficulty in such a place may be "spun around" by the stormy weather, because the keel of the ship is dragging on the bottom of the water but never becomes stuck fast in one place.
The image of the second stanza is of the bodies and souls of the victims being dragged about for ever as the sea tides wash back and forth over the shoals, rather than being buried "safely" in a grave on land.
You might want to do some research into notorious shoals and sandbanks that are navigation hazards even in fair weather, such as the Goodwin Sands near Dover in the UK, where more than 2000 ships are known to have been wrecked in an area just 10 miles long and 3 miles wide. For example in a single night in 1703, more than 50 ships and 2000 lives were lost in a storm. As the sandy sea bed is continually moved by the sea, sunken ships (and the remains of their crew) are sometimes uncovered after having been buried for many years.