The correct answer is C. From a starting position of 3 meters below sea level, the fish is descending 7 meters per second.
This is because in the equation y = -7x + 3, the coefficient of x is -7. This means that for every second the fish is swimming, its depth below sea level decreases by 7 meters. The positive constant 3 indicates that the fish starts at a depth of 3 meters below sea level.