I have developed an Apple Watch application using IBM MobileFirst Platform 6.3 in testing environment and it's working fine.
My clarification is to know, whether the same Apple Watch application development will work using IBM Worklight 6.0.
Worklight 6.0 was not tested with the Apple-supplied WatchKit, but there are no known issues preventing it from not working.
It should be noted that at this time IBM MobileFirst Platform Foundation (previously known as "Worklight") does not claim Apple Watch support.
You can go ahead and try to see if it is working. As you have already experienced, it does work with MobileFirst Platform Foundation 6.3. Why go back to Worklight 6.0? Upgrade your existing application instead (and there are very good reasons for that from 6.0.0.2).