It also goes by the names of:
While this film has been heavily criticized for being "too cheesy" for lack of other descriptions, I personally enjoyed it. As of the time of this question, it's still available on YouTube (though in several parts). Basically, the plot of this film is that an atomic bomb dropped into the sea creates a large sea serpent that goes on a rampage, killing anyone and destroying anything in its path. It's not very graphic, but I do believe The Sea Serpent warrants a PG-13 rating (if it had been rated).
IMDB didn't give a rating on the film, but since I've seen the movie, that's my best guess on the rating.
If you haven't seen the movie and are interested in seeing it (though I doubt many people will be interested in it), you can search for it on YouTube as long as it doesn't get deleted. Like I said, it's still there now on YouTube. Hopefully, it won't get deleted.
And...if you HAVE seen the movie, what did you think of it?