First things first, IRS Circular 230 requires that I say that nothing I advise should be used to commit fraud or avoid penalties. Talk to a tax professional about your specific situation.
The problem with the question of whether you can 'write it off' is that it depends on your tax situation, which will be unique to each person. The key question is how to consider the watch.
If it is defined as a part of your clothing (e.g. jewelry), then it is not deductible, period.
Secondarily, if it doesn't have a justifiable business use, then it'd be considered for amusement, and not be eligible for depreciation.
The next question depends whether you are self-employed, or are an employee. If you're an employee, then the question is whether it meets the two part test. First, it must be for your employer's convenience. Second, it must be required as a condition of your employment. Since the watch doesn't exist yet, I would be highly skeptical of a claim that it's required as a condition of employment.
Under Publication 946, "Listed Property" includes assets with both business and personal uses, like cars, computers, etc. As Listed Property, it could be eligible for depreciation, which could reach 100%. However, it would require a business use.
To depreciate the watch, you will have to depreciate the Apple Watch based on your business use %. To do that, you need a reasonable method to demonstrate how you allocate personal use vs. business use. Simply saying "Oh, i wore it to one meeting, boom, 100%" won't stand up in an audit.
If you're a programmer, then buying one for test purposes would absolutely qualify as a business use. However, you'd want to determine how much of the purchase was for pleasure/recreational purposes, and how much was developer related. Similarly, if you're a fitness professional, and use it as a fitness watch, then also, yes, you could deduct it as being related to your profession. However, again, you'd have to determine use %.
However, for most of us, I believe attempting to 'write it off' would be a very aggressive tax position.
Source:
http://www.irs.gov/publications/p946/ch05.html.
As always, consult a tax professional.