Mermaid Man Watch
The Mermaid Man Watch appeared in the episode Mermaid Man Begins.
The Mermaid Man-clock looks like the head of that superhero. When his mouth opens, you can read the time, represented in digital numbers.
So it looks like, you can use the clock as an alarm clock. He starts at the specified time, play a music and the phrase, "It's time, it's time, it's time for the battle against evil", of course with mermaid's voice. As he says this, his mouth open repeatedly in time with the music and the words.