I would guess the item is one of two things:
1) Something that might explode; a mine, a missile, a torpedo, somehow ended up in the water. You cannot leave explosives in the water, as they present a hazard in the future.
2) Something technically sensitive, which is why the "item" was not described.
Most sport divers in normal gear go to maybe 120 feet max, and prefer to keep it at 90 feet or less. Anything below 100 feet, and you risk nitrogen narcosis and decompression sickness.
200 feet is what is called "technical" diving and is for professionals only. They use special mixed gases, and must follow procedures exactly or they can get the bends. You can't just toss a few SCUBA divers overboard to tie a cable to it.