The captain of the South Korean ferry that sank with hundreds of high school students aboard is under criminal investigation for his actions, but it's not clear whether he broke any laws by being one of the first people off the crippled boat.

While the "captain goes down with his ship" is considered a law of the sea, it's really more just a guideline, experts say.

Lee Joon-seok, 69, climbed onto one of the first lifeboats to launch from the ship just a half an hour after reporting an accident.

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"I am really sorry and deeply ashamed," Joon-seok said today as he was being questioned at the Mokpo Coast Guard Office.

It is not the first time that authorities have focused on a captain's abandonment during an investigation. In 2012, when the Costa Concordia cruise ship sunk off the coast of Italy, Captain Francesco Schettino was brought up on criminal charges for abandoning the ship ahead of his passengers.

But is it against the law for captains to leave the boat while it goes under?

It depends.

There is no international maritime law that requires a captain to stay on a sinking ship, but many countries either have their own laws or subscribe to international treaties that mandate certain behavior.

South Korea, for instance, is a member of the International Maritime Organization which has its own rules for captains outlined in the International Convention for the Safety of Life at Sea. The convention doesn't mandate that the captain stay on board, but its rules suggest a captain is always responsible for the people on board.

"There is nothing in any IMO Convention to specifically require a captain to stay on board the vessel in the event of an incident such as this, however he/she does retain full responsibility for the safety of the vessel and those on board," IMO spokesman Lee Adamson told ABC News in an email today.

There are also guidelines presented in the Merchant Marine Officer's Handbook which say the captain should be the last person to leave the vessel, but the guidelines are just that, guidelines, not law.

If South Korea does not have its own laws that dictate a captain must stay on the ship, Joon-seok may not be charged criminally for leaving the vessel while his passengers were struggling to escape.

In all, the tradition of a captain going down with the ship may be more about personal choice and lore of the sea than legal responsibility.

In this handout provided by Donga Daily, The Republic of Korea Coast Guard work at the site of ferry sinking accident off the coast of Jindo Island on April 16, 2014 in Jindo-gun, South Korea. (Park Young-Chul-Donga Daily via Getty Images)