This movie was on when I was a kid so there's very little associated with the memory. There's a dock around a big lake and a boy goes exploring. During various parts of the movie he keeps seeing the lake water bubbling and there is some kind of monster or scary thing under the lake of which he is afraid. Does anyone know this movie or TV show? It would have been around mid 80s or early 90s. From various internet searches I've seen people say there was a bulldozer in it but I don't remember any of that.
This is Frog Dreaming (1986), also known as The Quest.
An American boy, Cody (Thomas), whose parents have died, lives in Australia with his guardian, Gaza. Cody is very imaginative, inventive, and inquisitive. He builds things in his garage, including a railbike which he uses to get around. Cody comes across some strange events happening in Devil's Knob national park associated with an Aboriginal myth about "Frog Dreamings" and Bunyips, terrifying water monsters that prey on humans. Cody tries to investigate. The occurrences revolve around a lake where a bunyip the locals call "Donkegin" supposedly lives.
After Cody witnesses the centre of the lake erupting in bubbles, he discovers the desiccated body of a homeless man, Neville, in a tent nearby. The local police investigate but determine only that Neville likely died of a heart attack. Determined to pursue the mystery of the pond himself, Cody fashions a makeshift diving suit and proceeds to explore the murky bottom, but never comes back up. Thinking that he has drowned, the townsfolk decide to drain the lake to recover his body. However, before they can finish, Cody's friend Wendy observes an air toy in Cody's aquarium, and a book on old mining equipment, and realising Cody may be alive, rallies aid to send a diver team into the pond. The diving team attempts to locate Cody and bring him an oxygen tank, but before they have a chance, the lake begins to bubble and seethe once more. Donkegin emerges with Cody in its jaws and raises its head in an unearthly cry, reminiscent of old, rusted metal. One of the officials recognizes the shape as lights penetrate the weeds and algae that cover Donkegin, giving it its monstrous appearance.
They discover that Donkegin is in fact an old donkey engine or a type of excavator or steam-shovel used in construction work years ago, and the lake is in fact a flooded quarry. It is also revealed that many items have accumulated at the bottom of the pond including a car, a bicycle, oil drums, and other assorted junk. The locals manage to get Cody out and to safety and dispel the myth of the monster in the water.