As shipowners seek to boost business and improve efficiency, marine insurance markets face mounting challenges that include larger vessels, skyrocketing salvage costs and risky Arctic shipping routes.

Some problems are interrelated: Huge new container and cruise ships not only create the potential for aggregation of cargo and hull risk but also massive salvage and wreck removal losses in the wake of an accident, marine experts say.

Two of the costliest wrecks on record, the cruise ship Costa Concordia and the container vessel MV Rena, were relatively small by recent shipbuilding standards.

The Costa Concordia, which has cost underwriters about $2 billion in hull and wreck removal losses, “could have been a lot worse if it had been a larger cruise ship,” said Jonathan Ball, international ocean marine manager at General Re Corp. in Stamford, Connecticut.

While marine liability rates have risen in response, cargo and hull underwriters are confronting the exposures in generally soft market conditions with competitive pricing and plentiful capacity.

“The market seems to be impervious to the effect of large losses,” said Peter Austen, New York-based CEO of Willis North America Inc.'s "North America marine division.

“Underwriters can have short memories,” said Richard DeSimone, president of XL Group P.L.C.'s North America marine unit in New York.

Commercial ships have been growing steadily for decades, but now have reached proportions that worry many underwriters.

As recently as 10 years ago, the largest containerships could carry boxes equaling about 8,000 20-foot equivalent units; last year, Copenhagen-based Maersk Group launched the first of its Triple E-class ships, which carry 18,000 TEUs and measure 1,312 feet long and 194 feet wide. Hong Kong-based China Shipping Container Lines is building five 19,000 TEU ships, and marine market sources say 24,000 TEU vessels are likely in the near future.

While the Costa Concordia, launched in 2005, measured 114,147 gross tons and could carry nearly 4,000 passengers, the largest cruise ships today weigh in at more than 225,000 gross tons and can carry more than 6,000 passengers.

Salvage and wreck removal costs have jumped in recent years, with even smaller ships presenting complex and time-consuming salvage problems.

The Rena, a 3,351 TEU ship, ran aground and broke up on a protected reef off New Zealand in 2011; as of June this year, salvors had recovered 77% of the containers and removed large sections of wreckage, but the ship's owner has applied to local authorities to leave what remains of the wreck underwater, according to The Swedish Club, the ship's protection and indemnity insurer.

Removal costs have totaled about $400 million so far, marine market sources say.

A number of factors account for the rising costs, including the complexity of salvage operations, pollution concerns and greater government involvement, Mr. Ball said.

While salvors once might have dragged away a wreck with relatively few concerns about pollution or left it as long as it wasn't a navigation hazard, governments now are far more focused on environmental risk and often insist on complete removal, several market sources say.

“Nobody wants one of these ships sitting off their shore,” Mr. Ball said.

The newest container ships compound these problems. For one thing, salvage companies have not yet invested in cranes and other equipment they need to offload thousands of containers and then deal with a disabled megaship, experts say.

“The equipment doesn't exist,” said Michael Kingston, associate partner and marine specialist at law firm DWF L.L.P. in London. “The technology is there, but nobody is paying for the equipment.”

Even if such equipment were available, the success and cost of a salvage operation still would depend on the remoteness of the wreck and how quickly equipment could reach it, as well as the time it would take to remove the massive number of containers, experts say.

Environmental damage also is greater with larger ships, which carry more fuel, while the ability to fight fires in the holds of megaships is still unknown, Mr. Ball said.

“We are really entering into somewhat of a new world” with the largest new ships, Mr. Austen said.

Given that an 18,000 TEU container ship could carry more than $1 billion in cargo on a single voyage, underwriters are also concerned about aggregation of risk: “Aggregation of values is a big problem,” Mr. Kingston said.

Catastrophe exposure from cargo accumulating in ports also is a challenge for underwriters.

“We don't always know where the goods we're insuring are,” said Drew Feldman, vice president and worldwide ocean marine manager at Chubb Corp. in Warren, New Jersey. “That's a significant challenge when you begin to consider the catastrophe risk” that includes hurricanes and earthquakes in ports such as Los Angeles or Kobe, Japan.

Without the sophisticated modeling tools that property insurers use, cargo insurers must look at the throughput of a given port, the market share of an insured cargo owner and the average value per container-load to measure their potential catastrophe exposure, Mr. Feldman said.

“That's multiple levels of educated guesswork and supposition,” he said.

The risk is heightened by the arrival of megacontainer ships: “All it does is increase the accumulation of values at these ports subject to catastrophe risk,” Mr. Feldman said.

Tracking of individual cargoes, driven by the shipping logistics industry, is likely to improve in coming years to the point where a given cargo's location on a 20,000 TEU vessel could be pinpointed, he said.

Meanwhile, the opening of Arctic waters as climate change reduces sea ice will bring more challenges to marine interests. The Arctic is expected to draw cargo ships, which can shave thousands of miles off current routes, as well as cruise lines, oil and gas drillers, and mining operations.

Shipping interests face unique risks in Arctic waters, though, including the relative lack of accurate charts, remoteness from ports, and the effect of ice and cold on ship systems, marine sources say.

The United Nations' International Maritime Organization is drafting a mandatory international Polar Code that will set requirements for ship design and construction, equipment and safety training, search and rescue, and environmental protection.