The weight dragged him down into the water.
It was terrible. He struggled heroically, and managed to get to the surface again.
Then he tried to escape it by taking to the air, but could not achieve liftoff. The lead sinker was heavy, and was weighing him down too much.
To make matters worse, it had stuck in his throat. This gave him a floatation problem.
Finally he figured out what to do. He took a deep breath, stuck his head in the water, waggled his tail violently. . .
. . . and he coughed.
This expelled the lead sinker from his throat.
"Ahhh!" he said. "That's MUCH better."
"That was quite a nasty surprise," he said as he explained the whole thing to me later. "But it had a rather interesting side-effect. I swallowed a large amount of air just before I coughed, and since then it's been going straight through me."
"Really?" I said. "How unpleasant."
"Oh, no, not really," said the duck. "It's quite useful, really."
"Useful?" I asked doubtfully. "How?"
"I'll show you," said the duck, and he did.
"See?" he said.
"I've become jet-propelled!"