Deflating the lungs would have been sufficient to sink the corpse, but Arnold was squeamish so he opted for concrete shoes. 

Nobody really tells you how to make concrete shoes. It may even be somewhat of a gangster myth. But Arnold didn't like knives or blood so he went to the hardware store and bought two five gallon buckets and two sacks of dry concrete. 

Now Arnold, though a mobster, was no killer. He was pretty low on the pecking order. In fact, many of the other members of the mob thought he was an idiot. But, as the nephew of the boss, he had to be included in any plan.

"Show him the ropes," the boss had ordered.

The others soon realized that Arnold was a hopeless case. He accidentally shot one of their own (in the toe) at a bank robbery, so they took away his gun. They tried to give him simple tasks he couldn't screw up. 

But when Arnold was around, things went poorly. He would set off the alarm, wake the guard dog, alert the sentry. He also was pretty bad at spilling the beans to the wrong person at the wrong time. In short, he was a liability.

"Arnold, dispose of this gentleman," said the boss. 

The other men looked at each other nervously. They had just strangled the 'gentleman,' and were already throwing him into the trunk of the car. They could easily get rid of him and they didn't even trust Arnold to take out the recycling properly. 

"We got this boss," they said.

"No!" said the boss. "You are coming with me. Leave the disposal to Arnold."

So they had no choice. 

Arnold began to sweat nervously as he always did under pressure, but he drove off alone with the corpse in the back of the car determined to please his uncle.

He pulled up to the deserted lakeside park well after midnight. He was pleased that he'd manage  to get the man's feet to actually stay in the buckets while the concrete set. What he didn't realize was how heavy the man and his new shoes would be to get out of the car and onto the rubber raft. 

He finally opted to carry the buckets by the handles and slowly dragged the body behind him over to the water's edge. He pumped up the raft and rolled the man in. The boat sat dangerously low in the water from all of the weight. Arnold waded out pulling the raft with a rope until the water got above his knees. Then he got in with a paddle. Dark waves lapped over the edge threatening to capize the flimsy vessel. But Arnold was determined. 

A dozen yards out he judged he was far enough. He lifted the man's torso bringing him face to face with his victim.

The man's eyes opened and Arnold screamed. They began to struggle. Metal flashed in the moonlight before the knife rammed between Arnold's ribs, punctured his lung. The two flailed about. The man stabbed Arnold again on the other side of the chest and Arnold went limp as the whole raft flipped over. The rope tangled in the handles of the bucket causing the overturned raft to anchor tightly to the bottom of the lake. 

Bubbles rose for a minute and then stopped. 

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A/N from a "prompt" or rather a speculation by my sister at the dog park one morning on how could an overturned rubber raft be anchored just off shore and somehow also involve a dead body.