At least it doesn't crash now, as long as 2 peers are connected.

godot_ship/script/game/Gameplay/Game.gd

@@ -37,15 +37,11 @@ func _ready():
 	game_setup()
 
 # Function used to keep track of which players are ready
-remote func player_ready(pid):
-	print (get_tree().is_network_server())
-	var who = pid
-	# Here are some checks you can do, for example
-	assert(get_tree().is_network_server())
-	assert(who in Network.peer_info) # Exists
-	assert(not who in players_ready) # Was not added yet
-
-	players_ready.append(who)
+mastersync func player_ready():
+	var who = get_tree().get_rpc_sender_id()
+	if get_tree().is_network_server() and who in Network.peer_info and not who in players_ready:
+		print ("ASSERT SUCCESS")
+		players_ready.append(who)
 
 	if players_ready.size() == Network.peer_info.size():
 		rpc("game_start")
@@ -58,11 +54,13 @@ func game_setup():
 		# TODO: Create a fake peer who we can automate, for single-player mode
 		Network.start_server()
 	network_id = Network.get_network_id()
+	var count = 0
 	# Create players for every player in Network.peer_info
 	for k in Network.peer_info.keys():
 		# Create a new player
 		var player = Player.instance()
 		# Set the player's opponent, for now
+		player.opponent_pid = Network.peer_info.keys()[1 - count]
 		# Give the player a recognizable name, like "1", instead of "@@97"
 		player.name = str(k)
 		# The player controls themselves
@@ -71,28 +69,28 @@ func game_setup():
 		players[k] = player
 		# Add the player to the scene tree
 		add_child(player)
+		count += 1
 	pass
-	
 	# Connect to your own player_ready signal
 	players[network_id].connect("player_ready", self, "_on_player_ready")
 	# Have your player set up the board:
 	players[network_id].set_up_begin()
 
-func game_start():
+mastersync func game_start():
 	# Make sure we're the server
 	assert(get_tree().is_network_server())
 	while not winner:
 		for id in players.keys():
+			# TODO: RPC always returns nothing.
+			# Figure out how to work around this.
 			var hit = players[id].rpc_id(id, "turn_start")
 			var result = players[hit["id"]].rpc_id(hit["id"], "hit", hit["target"])
 			players[id].rpc_id(id, "mark", hit["target"], result)
 		pass
 
-func _on_player_ready(pid):
+func _on_player_ready():
 	print ("_on_player_ready")
-	match pid:
-		1: player_ready(pid)
-		_: rpc("player_ready", pid)
+	rpc_id(1, "player_ready")
 
 #   victory_screen: display the victory screen
 func victory_screen():

godot_ship/script/game/Gameplay/Player.gd

@@ -24,7 +24,7 @@ func _ready():
 	pid = int(name)
 	board = Board.instance()
 
-remote func set_up_begin():
+mastersync func set_up_begin():
 	var setup = Setup.instance()
 	setup.connect("board_ready", self, "set_up")
 	add_child(setup)
@@ -32,13 +32,13 @@ remote func set_up_begin():
 # Member functions:
 #   hit: Called when opponent fires on us.
 #     Update internal state, and return hit/miss/sunk
-remote func hit(pos):
+mastersync func hit(pos):
 	var res = board.hit(pos)
 	return res
 
 #   mark: Called when the opponent returns hit/miss/sunk
 #     Update internal state, return ack/nak
-remote func mark(pos, value):
+mastersync func mark(pos, value):
 	# Mark the position on the top board
 	board.fire(pos, value)
 
@@ -57,12 +57,12 @@ func set_up(ships):
 		place_ship(i[0], i[1], i[2], i[3])
 	# Add the board to the tree
 	add_child(board)
-	emit_signal("player_ready", pid)
+	emit_signal("player_ready")
 
 #   turn_start: start player's turn
 #     Initiates the player's turn, and blocks until the player selects a location to fire upon
 #     returns: fire = [player id, target coordinates]
-remote func turn_start():
+mastersync func turn_start():
 	print("turn_start")
 	var fire = Fire.instance()