Longest time underwater

Drawdown depth is only half the risk story; investors also care about drawdown duration, the "time underwater." A strategy is underwater at time $j$ if its equity is strictly below the highest value reached before $j$. The longest underwater stretch is the maximum number of consecutive periods spent below a prior peak, ending when the curve makes a new high.

equity = [100, 120, 90, 105, 60, 80, 130]
-> 4             # days at 90, 105, 60, 80 are all below the peak of 120

Return the length of the longest underwater run, in $O(n)$ time and $O(1)$ space.