If you want to have an uninterrupted session of watching videos on youtube then the only option you have right now is to search beforehand for all the videos you would like to see and make a playlist of them and start watching.
But this is practically impossible as so many times we get an idea of some video while watching another video.
Right now Youtube does not support the feature of letting you search for videos while you are watching some video. The new search page that will open will obviously stop the currently playing video. If you choose to open a new tab then also your video session gets interrupted.
Best Chrome Extensions - QueueTube
QueueTube is a free chrome extension that will help you in situations like these. Install this extension and now you can search for more videos and at the same time continue playing the video you are watching. You'll see the search results in the right sidebar where usually related videos appear.
Now you can add the videos you like to a playlist while you are watching some other video.