see this is what I'm saying.. typically apple will list it if it was compatible... it was nowhere in the 12w power brick and ironically not even listed in the 5w LULZ.
I didn't bother to check the 5W charger, since that's what came in the box with the Watch. However, since the watch is not listed even on the 5W charger page, I'm guessing that it's either an oversight or the pages just haven't yet been updated.
Now, since I charge my devices at night (when I am not using them and don't anticipate a sudden need), I use the chargers that came with the devices to charge them. YMMV