How can I see all the issues I'm watching on Github?

You can see all the Github issues you are currently subscribed to at https://github.com/notifications/subscriptions

You can navigate to this page from any page by clicking the notification/bell icon on the top left and then selecting "Managed Notification" > "Subscriptions" from the left menu panel.

Github does not have any option to list all the watched issues.

Marking labels on such issues also does not solve the purpose.

But github sends notification whenever there is any change in the issue. So you can check all the notification at a single place https://github.com/notifications

By default, this will show unread notifications (also indicated by a mailbox with a number in the top right corner). From that page you can choose "All Notifications", or https://github.com/notifications?all=1 to see all the issues being watched that have had at least one update since you subscribed to it.

According to the GitHub API v3 documentation¹, there is a way to list subscribed issues in owned repositories, member repositories, and organization repositories. However, it does not list subscribed issues from any arbitrary repository in which you are not involved.

On Unix you can access the API like this (just enter your GitHub password when propmted):

curl --user "MyUserName" https://api.github.com/issues?filter=subscribed

Output:
[
  {
    "url": "https://api.github.com/repos/owner1/repoA/issues/3",
    "repository_url": "https://api.github.com/repos/owner1/repoA",
...etc...

Or use this command to format the output as a list of links to the issues:

curl --user "MyUserName" https://api.github.com/issues?filter=subscribed | \
    grep '"url"' | grep -o 'https://api.github.com/repos/.*/issues/[0-9]*' | \
    sed 's#https://api.github.com/repos/#https://github.com/#'

Output:
https://github.com/owner1/repoA/issues/3
https://github.com/owner1/repoB/issues/14
https://github.com/owner2/repoC/issues/1

¹ Since my edit to the first answer mentioning the GitHub API was rejected, I'm adding the examples here.

---

The following method does not work for subscribe-only issues.

As a workaround you can enter this into the search box, either on https://github.com/, or on https://github.com/issues/

is:open is:issue involves:YourUserName

This will show you all issues in which you are involved in some way, but not issues you are only subscribed to. The GitHub help page states:

The involves qualifier is just a logical OR between the author, assignee, mentions and commenter qualifiers for the same user.