There were actually a few threads on this topic.
The iPhones, iPads and Apple Watches all have a consistent, throttled draw. It doesn't matter if you put them on a more robust charger - they won't charge any quicker.
That doesn't appear to be the case, at least with the iPhone 6. I haven't done any proper testing, but it does seem to charge faster with the iPad charger, and this is born out by things like this:
https://www.macrumors.com/2014/09/22/iphone-6-and-6-plus-charging/
I don't know whether this would carry over to the watch in terms of charging faster, but I've been using an iPad charger from the very beginning with no ill effects.
That doesn't appear to be the case, at least with the iPhone 6. I haven't done any proper testing, but it does seem to charge faster with the iPad charger, and this is born out by things like this:
https://www.macrumors.com/2014/09/22/iphone-6-and-6-plus-charging/
I don't know whether this would carry over to the watch in terms of charging faster, but I've been using an iPad charger from the very beginning with no ill effects.
FYI, started to use a 12W iPad charger block last night attached to a standard (metal) Watch cable and the round cable end started getting hot. I can't think this is a good thing - haven't noticed this with the supplied power block and I stopped using the 12W block immediately.
I had the same thing happen to me. The 12W made the charger get extremely warm.