This solution does one pass and gives all the water between all the puddles found in the shape given

FuzzBizz

shape = [1, 0, 3, 4, 5, 3, 2, 4, 6, 8]
puddles = [0]
current_puddle = 0

left = nil
right = nil
has_right_wall = false

shape.each do |height|
  left = height if not left
  right = height
  
  if right < left
    puddles[current_puddle] += left - right
    has_right_wall = false
  else
    current_puddle += 1
    puddles[current_puddle] = 0
    left = right
    has_right_wall = true
  end
end

unless has_right_wall
  puddles[current_puddle] = 0
end

p puddles.reduce(0) {|memo, water| memo += water; memo}