From what I could find elsewhere, it looks like you can only get AppleCare+ if the specific watch in question is still within the original 60 days from first purchase. I haven't been up to Best Buy, as the refurbished sale price jumped from $103 off the list price to only $34 off. For that small of a difference, I'd rather not risk being unable to buy insurance on it.
It did also strike me as odd that online at least, Best Buy doesn't try selling you their own extended warranty. They've offered it on other items, such as an iPod I bought for a child that didn't yet need a phone. Granted that was also a new item, and not a refurbished, but I'm not sure that was the reason. If Geek Squad is willing to certify a refurb, they likely should offer an extended warranty.