apple id on apple watch

Unfortunately the only way to change the watch Apple ID is from the Watch app on the paired iPhone. I have no idea whether that would cause a conflict with your father’s ID already on the iPhone or not, and I’m not sure anyone here has tried what you are scheming.

I’m also unsure how the pairing to your father’s phone would impact stuff like email and iMessage traffic. I suspect this would still sync via Bluetooth to your dad’s phone if it was within range, and I can’t see a way to transfer the link to your iPad.

That would be pointless, and very difficult or impossible to setup.