Maps may be bookmarked in couple of ways. First, you can simply open the tools menu (by clicking on the tools icon in the top left corner of any project) and click "bookmark". 

This will create a URL that you may copy and share with others via email or some other application. Anyone who clicks on that URL will be brought into SeaSketch, with the map extent and layers showing when you created the bookmark. 

You can also share map bookmarks in discussion forums. When posting a message to a forum bookmarks to your map extent, base map and active data layers are included with your message. That way, anyone who reads your message (including you!) will have access to that bookmark. Here is a video demonstration of posting a message that includes a map bookmark: