A boat - moving or not?

Look at the following video clip.

Can you tell if the boat is moving or not?

Video 2.2: Riddle 2

1. Why was it difficult for you to tell if the boat was moving or not? 

2. In order to say that something is in motion, do we need to refer to surrounding object(s), tree, water, other boats, etc.?

3. Along with the surrounding object(s), does the time also help in saying if something is in motion or not?