@boomerang,
I think they can be sort of similar, but nowhere near the same.
I love both.....web surfing is a bit like day-dreaming, because it is often associative...you know, when you see something that piques your interest, then in reading about that, you become interested in something else (I actually had a thread about that process, thanking the techies who made it possible), and so on, until you find yourself miles from where you began, and having learned and enjoyed a heap of stuff on the way.
But I think it way more cognitive (although guided to some extent by unconscious processes, I suppose).
I think day-dreaming is far more an unconscious process...with the cognitive bits as the tip of the iceberg, of course...and far more emotionally than intellectually satisfying.
Though, of course, reverie often begins with a cognitive push, and there is a lot of cognitive processes going along.
I guess one, to me, is like wandering a city, and the other more like floating down a stream.
I think both are great, and neither a waste of time...unless they are all you do, or are done when you are putting off tackling an important task.
I suspect both tap into the crucial "alert wakefulness" state of our baby brains, and that a great deal of learning and sorting and reflecting is done during these activities.
Reflective capacity is a key marker and creator of emotional and cognitive health.