Well that's good that even if he's not speaking he'll appear. Maybe they'll make a joke about Mermaid Man having laryngitis at least once.
On a side note, I wonder what would happen if one of the main crew members suddenly passed away. Would they be as willing to retire that character? I say no because even though they would show the actor a lot of respect, the show must go on. And Mermaid Man was more of a recurring character.