water.js

var arr = [2,1,5,0,3,4,7,2,3,1,0];

console.log('waterRecursion', findWater(arr, Math.min(...arr), Math.max(...arr)));

///////////////////////////////////////////////////////////////////////////////

var depth = Math.min(...arr);
var maxHigh = Math.max(...arr);
var water = 0;

while (depth <= maxHigh) {
    for (var idx = 0; idx < arr.length; idx++) {
        if (arr[idx] <= depth) {
            if (checkForWater(idx, depth, arr)) {
                water++;
            }
        }
    }
    depth++;
}

console.log('water', water);

///////////////////////////////////////////////////////////////////////////////

function checkForWater(idx, depth, arr) {
    for (var i = idx; i < arr.length; i++) {
        if (arr[i] > depth) {
            for (var j = idx; j >= 0; j--) {
                if (arr[j] > depth) {
                    return true;
                }
            }
            break;
        }
    }
    return false;
}

function findWater(arr, depth, maxHigh) {
    var res = arr.reduce(function (total, currentValue, currentIndex, arr) {
        if (currentValue <= depth) {
            var testDepth = function (el) {
                return el > depth;
            };
            var testLeft = arr.slice(0, currentIndex + 1).some(testDepth);
            var testRight = arr.slice(currentIndex).some(testDepth);

            if (testLeft && testRight) {
                return total + 1;
            }
        }
        return total;
    }, 0);

    if (depth < maxHigh) {
        return res + findWater(arr, depth + 1, maxHigh);
    }
    return res;
}