Semantics should come into play when building emails as we are using HTML and CSS. Using the <strong> tag will aid in accessibility (e.g. screen readers)
The basic rule of thumb is if you want to stress the importance of the text, use <strong>. If you want to style text to be bold, use the <b> tag.
Both tags should be supported across the board as these are basic markup tags. Sorry if I cannot help you even more.
If this is a one off campaign then bitly.com could be your answer.
If this is a weekly or monthly ongoing newsletter then I would recommend contacting your ESP (Email Service Provider) and see what internal tools you can utilise.
e.g. Mailchimp has tracking and comes with click maps. There is also another option to integrate Google Analytics.
I am pretty sure the Apple watch will not support HTML emails with its native email app. Instead they will render plain text just like Android wear.
I would recommend focusing on plain text and the subject line will be very crucial on such a small device.