I saw a movie when I was really young about a mermaid with blonde hair. All I really remember about it was that she had blonde hair, part of it took place on a ship, and I think there was a scene where the mermaid had a knife in her hand and may or may not have killed someone. I know that sounds strange. The movie was a cartoon, and I think it had mermaid in the title, but of course I could be wrong. Does anyone have any idea what movie I'm talking about?

You're actually thinking of a Little Mermaid movie. Not the Disney one of course but another one. I know this because of the knife. In the original Little Mermaid story, in order for the little mermaid to remain alive she had to kill the prince. Her sisters sold their hair to the sea witch for the knife. The little mermaid goes to kill the prince with the knife but is not able to go through with it and throws herself into the depths of the sea to become sea foam (or depending on the version a sea spirit or something less…foamy).

You may be thinking of this version: The Little Mermaid (1993) (V) (http://www.imdb.com/title/tt0216882/)
There are many other adaptations but most are short 30min.