It doesn't have any syntax errors, so I guess there's only one way to find out if it actually works :P

worldeditadditions/depends.txt

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+worldedit

worldeditadditions/floodfill.lua

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+--- Flood-fill command for complex lakes etc.
+-- @module worldeditadditions.floodfill
+
+local Queue = require "queue"
+
+local function floodfill(start_pos, radius, replace_node)
+	-- Calculate the area we want to modify
+	local pos1, pos2 = centre_pos
+	pos1.x = pos1.x - radius
+	pos1.z = pos1.z - radius
+	pos2.x = pos2.x + radius
+	pos2.y = pos2.y + radius
+	pos2.z = pos2.z + radius
+	pos1, pos2 = worldedit.sort_pos(pos1, pos2) -- Just in case
+	
+	-- Fetch the nodes in the specified area
+	local manip, area = worldedit.manip_helpers.init()
+	local data = manip:get_data()
+	
+	-- Setup for the floodfill operation itself
+	local start_pos_index = area:index(start_pos.x, start_pos.y, start_pos.z);
+	
+	local search_id = data[start_pos_index]
+	local replace_id = minetest.get_content_id(replace_node)
+	
+	local count = 0
+	local remaining_nodes = Queue.new()
+	Queue.enqueue(remaining_nodes, start_pos_index)
+	
+	-- Do the floodfill
+	while Queue.is_empty(remaining_nodes) == false do
+		local cur = Queue.dequeue(remaining_nodes)
+		
+		-- TODO: Check distance from start_pos
+		
+		-- Replace this node
+		data[cur] = replace_id
+		count = count + 1
+			
+		-- Check all the nearby nodes
+		-- We don't need to go upwards here, since we're filling in lake-style
+		if data[cur + 1] == search_id then -- +X
+			Queue.enqueue(remaining_nodes, cur + 1)
+		end
+		if data[cur - 1] == search_id then -- -X
+			Queue.enqueue(remaining_nodes, cur - 1)
+		end
+		if data[cur + area.zstride] == search_id then -- +Z
+			Queue.enqueue(remaining_nodes, cur + area.zstride)
+		end
+		if data[cur - area.zstride] == search_id then -- -Z
+			Queue.enqueue(remaining_nodes, cur - area.zstride)
+		end
+		if data[cur - area.ystride] == search_id then -- -Y
+			Queue.enqueue(remaining_nodes, cur - area.ystride)
+		end
+	end
+	
+	-- Save the modified nodes back to disk & return
+	worldedit.manip_helpers.finish(manip, data)
+	
+	return count
+end
+
+
+return floodfill

worldeditadditions/init.lua

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+--- WorldEditAdditions
+-- @module worldeditadditions
+-- @release 0.1
+-- @copyright 2018 Starbeamrainbowlabs
+-- @license Mozilla Public License, 2.0
+-- @author Starbeamrainbowlabs
+
+worldedit.floodfill = require "floodfill"

worldeditadditions/queue.lua

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+--- A Queue implementation, taken & adapted from https://www.lua.org/pil/11.4.html
+-- @module worldeditadditions.utils.queue
+
+local Queue = {}
+function Queue.new()
+	return { first = 0, last = -1 }
+end
+
+function Queue.enqueue(queue, value)
+	local new_last = queue.last + 1
+	queue.last = new_last
+	queue[new_last] = value
+end
+
+function Queue.is_empty(queue)
+	return new_first > queue.last
+end
+
+function Queue.dequeue(queue)
+	local first = queue.first
+	if Queue.is_empty(queue) then
+		error("Error: The queue is empty!")
+	end
+	
+	local value = queue[first]
+	queue[first] = nil -- Help the garbage collector out
+	list.first = first + 1
+	return value
+end
+
+return Queue

worldeditadditions_commands/depends.txt

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+worldeditadditions

worldeditadditions_commands/init.lua

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+--- WorldEditAdditions-ChatCommands
+-- @module worldeditadditions_commands
+-- @release 0.1
+-- @copyright 2018 Starbeamrainbowlabs
+-- @license Mozilla Public License, 2.0
+-- @author Starbeamrainbowlabs
+
+minetest.register_chatcommand("/floodfill", {
+	params = "<replace_node> [<radius>]",
+	description = "Floods all connected nodes of the same type starting at pos1 with <replace_node>, in a box-shape with a radius of <radius>, which defaults to 50.",
+	privs = { worldedit = true },
+	-- TODO: Integrate will the safe_region feature of regular worldedit
+	func = function(name, params_text)
+		local found, _, replace_node, radius = params_text:find("([a-z:_\\-]+)%s+([0-9]+)")
+		if found == nil then
+			found, _, replace_node = params_text:find("([a-z:_\\-]+)")
+			radius = 50
+		end
+		if found == nil then
+			replace_node = "default:water_source"
+		end
+		
+		local nodes_replaced = worldedit.floodfill(worldedit.pos1[name], radius, replace_node)
+		
+		worldedit.player_notify(name, nodes_replaced .. " replaced")
+		minetest.log("action", name .. "used floodfill at (" .. worldedit.pos1.x .. "," .. worldedit.pos1.y .. "," .. worldedit.pos1.z .. "), replacing " .. nodes_replaced .. " nodes")
+	end
+})