Hey,
this looks as if the client you are using does not load the watcher plugin. Can you show code, where you instantiate the client that you wrap around WatcherClient
Settings settings = Settings.builder().put("plugin.types", WatcherPlugin.class.getName()).build();
TransportClient client = TransportClient.builder().settings(settings).build();
client.addTransportAddress(new InetSocketTransportAddress(InetAddress.getByName("localhost"), 9300));
WatcherClient watcherClient = new WatcherClient(client);
in addition you can try the above snippet and see if it works.
--Alex