I saw Creature from the Black Lagoon as a young boy, when it first came out -- scared me away from monster movies to this day. ;-)
Little did I realize it was actually scientific research for future evolutionary changes:
“Reptilicus” did it for me.
For years, every time some scientist dug up something from the ice or tundra, I was filled with dread.
And then there was The Day Of The Triffids.
Once I got over the fear of killer plants, Children Of The Corn came around and my family’s crops freaked me out.
:D
I agree... saw it in the theater as a young boy and even though the monster was shown on the theater posters it was a startling intro on the screen. They set the scene up perfectly, slow and placid boat on the lagoon and then the creature burst up from the water!
Still one of the scariest monster intros ever... maybe even the most.