I'm pretty sure Apple Watch would've been called iWatch if it hadn't for the fact that at least two entities already have an iWatch trademark secured with the US Patent and Trademark office, and likely were asking a whole ton of money to release the rights.
This one, from 2008, looks an awful lot like how Apple stylized their trademarks back in the '90 and early 2000s, and specifically covers pretty pretty much everything: watches, clocks, watch bands, watches that play video games, cases, clasps, crowns and dials.
And this one, from 2014, is pretty obvious as to the intent: "
Goods and Services IC 009. US 021 023 026 036 038. G & S: Mobile phones; Devices for hands-free use of mobile phones; Headsets for cellular or mobile phones; Keyboards for mobile phones; Cell phone battery chargers for use in vehicles; Cell phone battery chargers; Portable and handheld digital electronic devices, namely, mobile phones and cell phones, for recording, organizing, transmitting, manipulating, and reviewing text, data, audio and video files
IC 038. US 100 101 104. G & S: Communications by mobile phones; Mobile telephone services"
I think Apple got a pretty hefty price tag to take ownership of these trademarks and went with Apple watch instead. No doubt internationally people and been registering watch trademarks in anticipation, expecting big paydays, and so it rapidly became no worth it.
Since Apple already owns the trademark to Apple and the Apple logo, it's legally very difficult for some speculator to start trademarking "Apple-" anything without an expensive legal fight. But Apple can't trademark the letter "i" as easily.
Anyway, Apple DID pay a lot of money to Cisco, and a few other companies internationally, to secure the rights to the iPhone trademark. Those negotiations weren't as expensive because no one at the time fully knew how wildly successful the iPhone was going to be (in fact, a lot of people expected it to flop, or at least stay a niche market). So i think they're going to keep their investment and stick to the name. If not, it would be quite a waste of money and legal resources.