☑ Boy carves Initials onto turtle shell; years later, turtle is gigantic

Years ago (late 80’s to early 90’s), we watched a movie in which a seaside town is terrorized by a sea-monster which turns out to be a giant turtle. At the very end of the movie, the turtle sinks into the ocean and we see its shell with the initials (with the typical ‘+’ and heart, I think) that the boy and girl carved onto it when they were young.

I tried Googling it, but the only thing I could find were some news articles about a real-life incident (from May 2012) where a turtle is found 47 years after a boy carved his initials in its shell. Obviously the movie predates the real-life incident (strange that I thought of the movie only a few weeks after the news broke).

Does anyone know what the movie was?