there are several optimal solutions that can be used to get identical objective function value.
Therefore, a scenario whereby the optimal objective function contour line coincides with one of the binding constraint lines on the boundary of the feasible region will lead to alternative optimal solution.
I believe he dove 6 feet because started 3 feet below water and went down to 9 feet below water and the difference between 9 feet and 3 feet is 6 feet.
