I remember a movie when I was a kid, there was a lake inside which there was a huge monster creature. It used to come out a little bit when two children used to come around the lake and there was a huge windmill which used to starts by itself when there was the time for the monster to come out. They also used to show one owl and a snake at the same time. The boy went inside the lake to find out what exactly is coming out from the lake and didn't come back for sometime. And the girl kept on waiting for him until he was back. Please help me with the name. Might be in 80s or early 90s.

Found this snippet from another site, it matches your description:
....A pond sits still at the bottom of a quarry as a man drifts lazily in a canoe. Suddenly the windmill starts spinning without the assistance of mother nature's farts. The blades spin faster and faster as violent bubbles emerge to the surface. The man violently paddles towards shore as the waves become intense from what stirs below the surface. As he climbs to land a creature breaks the surface causing the man the die in shock. The lake returns peaceful as the blades of the windmill slow to a halt....

You're looking for: The Quest, aka Frog Dreaming, aka Go Kids
Here's the opening scene:
http://www.youtube.com/watch?v=ISSYUSupBuE

found this snippet from another site, it matches your description:
....A pond sits still at the bottom of a quarry as a man drifts lazily in a canoe. Suddenly the windmill starts spinning without the assistance of mother nature's farts. The blades spin faster and faster as violent bubbles emerge to the surface. The man violently paddles towards shore as the waves become intense from what stirs below the surface. As he climbs to land a creature breaks the surface causing the man the die in shock. The lake returns peaceful as the blades of the windmill slow to a halt....

You're looking for: The Quest, aka Frog Dreaming, aka Go Kids
here's the opening scene:
Opening scene to The Quest, Frog Dreaming, The Go Kids - YouTube (http://www.youtube.com/watch?v=ISSYUSupBuE)

Thanks a million. That's it. Thank you so much. Was really looking for this movie for so long. You made it really easy. Thanks once again.

Great :)