“… by flooding tanks with water to dive and pumping water out with air to come back toward the surface.

An additional possible hazard for the vessel would be becoming fouled — hung up on a piece of wreckage that could keep it from being able to return to the surface.

If the submersible is found on the bottom, the extreme depths involved limit the possible means for rescue…. The Titanic lies in about 14,000 feet of water in the North Atlantic…. The only likely rescue would come from an uncrewed vehicle — essentially an underwater drone…. For recovering objects off the sea floor in deeper water, the Navy relies on what it calls remote-operated vehicles…. [such as] CURV-21 [which] can reach depths of 20,000 feet.

Getting the right kind of equipment… to the site takes time, starting with getting it to a ship capable of delivering it to the site…. In many submersibles, the air inside is recycled — carbon dioxide is removed and oxygen is added….

If the Titan’s batteries run down and are no longer able to run heaters that keep the occupants warm in the freezing deep, the people inside can become hypothermic and the situation eventually becomes unsurvivable. Should the submersible’s pressure hull fail, the end for those inside would be certain and quick….”

“The only likely rescue”… seems to mean the only remotely possible rescue.
ADDED: WaPo reports that “OceanGate and government officials had not released the names of the passengers as of Tuesday morning,” but that there were a “commander” along with 4 individuals called “‘mission specialists, who paid to take part in the expedition.”
We do know the name Hamish Harding, “a British businessman and seasoned adventurer,” because he wrote this on social media before he descended:
“This mission is likely to be the first and only manned mission to the Titanic in 2023.” 
Here, “likely” became certain. 
Unlike the NYT, WaPo allows comments. Empathy is not in the forefront:
1. “Re: four ‘mission specialists’ who paid to take part in the expedition/They are passengers, tourists. The Post doesn’t have to repeat the PR/advertising language of the company.”
2. “I understand that training was required of all passengers.”
3. “Their ‘specialty’ was having $250,000 to blow on a vacay.” 
4. “Let’s see. I’m going to pay an ungodly amount of money, enough to support a small family for a decade, to load my son and me into a tin can that is bolted closed from the outside, then dropped on a cable more than two miles into the freezing Atlantic ocean, far deeper than the depths that military submarines venture, at pressures up to 400 times that of sea level to see a sunken ship that has been extensively photographed, filmed, and analyzed, thereby giving you a far better intellectual experience at the local library or even in your home. Yes, Virginia, there is one thing money cannot buy: brains.” 
5. “But seeing it from the comfort of your own home doesn’t give you bragging rights at the billionaires’ club meetings….”

ALSO: 

“We run the whole thing with this game controller.”

AND: NYT: “The Coast Guard said visibility in the area where the submersible was lost was increasing today, after heavy fog yesterday. ‘If it’s on the surface, we’re fairly sure we’re going to be able to find it,’ a spokesman said.”