The fast charge function should work as designed and will not put your watch at risk because all Apple power adaptors are rated for wide range (100-230v) input power voltage. The only thing you might need will be a (dumb) plug adaptor to make the US plug physically fit sockets in India.
However, the North America cellular system uses different frequency bands to everywhere else in the world, so a USA specification GPS+Cellular watch would never be able to be activated or connect on Indian cellular networks. It would still work like the GPS-only versions when paired and linked by Bluetooth in range of your iPhone, but never standalone cellular. In other words it would be wasted money to buy the US cellular version.