Starting at the surface of the water, Dylan swims 4 feet below the surface. Then he swims down another 712 feet.

Answer 1

712+4= 716 feet below the surface

that's getting near Davy Jones' locker in some water areas, or in any waters, if Dylan can't hold his breath very long. Or if a shark or two lurk in the deep. 716 feet = 716/5280 = about 14% of a mile, or 2 1/2 football field lengths. An oxygen tank might come in useful, and a weapon to shoot sharks, or one of those cages to hide in, plus a friend in the boat above to pull you up if things go South.