The Case of the Horrible Swamp Monster by Drew Stevenson, 1984.
This is the first book in the Monster Hunter series.
Raymond Almond knows that his class project isn’t going to go well when he finds out that bossy Verna Wilkes is in his group. Verna convinces the group that their project should be a movie because her father recently gave her a movie camera. What makes it bad is that she insists that it should be a monster movie and that Raymond should play the part of the monster.
Raymond is embarrassed about having to stumble around the spooky Lost Swamp in the ridiculous monster costume, but he puts aside his embarrassment when what looks like a real swamp monster appears in the background of the footage. Even bossy Verna is scared at the idea of a monster living in the swamp where they were filming.
Fortunately, Raymond’s best friend is J. Huntley English, an intelligent boy and amateur cryptozoologist (or Monster Hunter, as he likes to put it). When Raymond and Verna tell Huntley what they’ve seen, he’s quick to join in the investigation, eager to discover proof of a real monster.
The course of their investigation takes a strange turn when friendly Mr. Walton, who works as a janitor at the same bank where Huntley and Verna’s mothers work, is arrested for stealing money. The kids refuse to believe that he’s guilty and soon learn that there is a connection between the thefts and the mysterious monster and possibly the strange fisherman who has also been lurking around the swamp.