When you ping a public IP address it will route it if you have set up failover… that is the aim of failover!
If you do netwatch in failover mode to check if “something is unplugged” then ping local IP address of the box (like upstream router).
If you ping a public address it will find it (you have a generic distance 2 route via 192.168…).
You can also do the recursive GW definition as in this post:
http://forum.mikrotik.com/t/advanced-routing-failover-without-scripting/136599/1
Question: Why would you need the netwatch script? It seems doing unnecessary things in my opinion which can be done directly with routing table…