Nearly four years after passage of a comprehensive cruise safety law, the U.S. Coast Guard is getting ready to propose a rule for three of the trickiest provisions in the legislation.

The proposed rule, expected out in June, will outline how cruise lines can comply with a requirement in the Cruise Vessel Safety and Security Act that they deploy technology for “capturing images of passengers or detecting passengers who have fallen overboard.”

Also addressed in the rule will be how a cruise ship’s video surveillance system should be operated to document crimes on the ship and assist in their later prosecution.

Advocates of the law say the provisions will make passengers feel more secure about taking a cruise.

But the cruise industry has raised red flags about the cost and practicality of applying the law. In one instance, a cruise line told auditors at the Government Accountability Office (GAO) that just doubling the time it keeps video footage from 14 to 28 days would cost an extra $21.8 million.

Most of the 15 provisions in the 2010 act, such as peepholes in cabin doors and standard rail heights, had been translated into Coast Guard guidance by June 2011.

The remaining ones involved complex technologies and, in some cases, language in the law that didn’t set a clear benchmark for gauging compliance.

In the case of detecting when persons fall overboard, the law called for compliance “to the extent that such technology is available.”

The Coast Guard asked for input from cruise lines, CLIA and passenger advocacy groups to help formulate its rule.

A recent report by the Miami Herald found that at least 28 passengers went overboard on cruise ships between October 2010 and June 2013. Rapid detection of persons going overboard would clearly enhance safety, but cruise lines say the jury is still out on automated detection systems.

CLIA told the Coast Guard that the technology to capture images of overboard episodes exists, but not the ability to detect them in real time. CLIA said vessel movement, sun glare, salt spray encrustation and weather all make instruments unreliable.

If detectors either fail to report incidents, or report false incidents, that would raise the cost, liability and burden on passengers.

Still, several cruise lines are testing various technologies. One unidentified cruise executive told the GAO that if companies are required to go to the expense of installing detectors, they should not produce inaccurate results that would increase operating costs.

Video surveillance is another area where technology exists, but its application is open for debate. Victim advocacy groups say existing cameras should be monitored continuously for crime, and recorded images should be stored for up to 90 days.

CLIA recommended a risk-based approach that would take into account differences among vessels, cruise lines and itineraries and said video should be stored for a week, nearly matching the average cruise length of 7.2 days.

Coast Guard officials told GAO auditors that when the rule is issued in June, rather than being prescriptive it is likely to be performance-based — outlining what is to be achieved — to allow for some flexibility in implementation.