IN the next several years, thousands of offshore oil and gas drilling rigs, many of them built during a global construction boom in the 1970s and ’80s, will reach retirement age and require decommissioning. Countries will have to decide whether to sink, remove or repurpose them.

While few proposals have been put in practice, there is no shortage of ideas for alternative uses of the platforms: supermax prisons, private homes, scuba schools, fish farms, windmill stations.

Unlike earlier generations of offshore rigs, which tended to be fewer, smaller and closer to shore, the ones being retired now are bigger, more numerous and spread much more broadly across the globe. Most of these retirement-ready platforms are too old for heavy industrial use, like drilling, but not necessarily old enough to demand full removal.

Oil and gas companies generally prefer to sink the rigs because it’s the cheapest solution. Many scientists back this approach, arguing that it creates underwater marine habitats and is less carbon-intensive than removing the platforms. Just renting a barge with a crane to pull the rig apart for scrapping can cost more than $500,000 a day. Eager to attract the tax revenue and investment often created by drilling, lawmakers in California have pushed a bill in recent years to fast-track approval for oil companies seeking to sink rigs.

But critics contend that many repurposed platforms bring more harm than good. Sinking rigs to turn them into reefs, for example, does not actually promote aquatic life, they say, but just attracts fish, concentrating them for easier capture. The metal on the structures, some spanning the space of a football field, rusts over time and can leach dangerous pollution, according to marine scientists. “Oceans shouldn’t be junkyards,” said Richard Charter, a senior fellow at the Ocean Foundation, a marine research and advocacy organization.

The waters here off the coast of Malaysia are being watched especially closely because many of the platforms here were built around the same time, more than two decades ago. More than 400 are due for immediate removal.

A couple of days spent on a converted platform last year in the Celebes Sea, a half mile off the coast of Malaysian Borneo, offered me a sense of the range of possibilities and challenges. “There are no mosquitoes, no flies, no sand to get in your gear and no tiring yourself out by lugging around equipment before or after the dive,” said Suzette Harris, managing director of the Seaventures Dive Rig, during my visit to her hotel and scuba school — a place of breathtaking beauty — located on the platform.

But the sea can be a lawless place and many of the world’s drilling platforms are in remote, sometimes dangerous waters. It took nearly 24 hours to get to Seaventures from a major airport (including travel by bus, propeller plane and speedboat). Because of recent kidnappings and attacks in the area by a Filipino guerrilla group, State Department officials warned me and a photographer against visiting this section of Borneo. The Malaysian military quietly posted armed commandos on the Seaventures rig at night to ensure no guests were kidnapped. As it happened, our trip was without incident; plenty of foreign tourists were also in the area.

Elsewhere, other groups are eyeing these aging platforms. An architectural organization based in London hosted a competition several years ago calling for design plans to build a prison on new or refurbished platforms. “Sea-steaders” have proposed buying platforms to create offshore communities. Their hope is to escape urban noise, crowds, crime and pollution, or potentially to move beyond the reach of certain laws and taxes in international waters.

In the United States, a federal rigs-to-reefs program has overseen the sinking of hundreds of platforms in American waters. Thousands more are coming up for decommissioning in the Gulf of Mexico over the next several years.

The federal program allows some oil and gas companies to convert their decommissioned rigs to reefs instead of requiring the companies to remove them. If the rigs-to-reefs option is expected to be less expensive than removal, the platform owner pays half the estimated savings to the state agency receiving the former platform. In Louisiana, the program is very popular among anglers because the artificial reefs attract fish. It has been opposed, however, by the Louisiana Shrimp Association, because the sunken rigs prevent shrimp boats from trawling the seafloor.

SUPPORTERS of the program say that aside from environmental benefits, sinking rigs helps keep energy prices lower for consumers by making drilling more affordable. “That’s exactly the problem,” said John Hocevar, oceans campaign director for Greenpeace. By saving drilling companies money, he said, rigs-to-reefs programs only promote more drilling, hide the true costs of this type of energy extraction and deepen overall dependence on fossil fuels. The repurposing of platforms built to be temporary also imposes long-term maintenance and liability burdens on the public, he added, enabling the companies that originally profited from them to avoid responsibility for their ultimate fate.

Dr. Sharul Sham bin Dol, the director of the petroleum engineering department at Curtin University in Sarawak, Malaysia, said that the platforms could work extremely well for fish hatcheries. “As fish cages hang down into the water below, they also reduce transportation and operational costs,” he said.

Dr. Ulugbek Azimov, professor of mechanical engineering at Northumbria University in Newcastle-upon-Tyne, England, added that the biggest advantage to moving onshore aquaculture to offshore oil rigs is that it would cut down on diseases and the accumulation of waste, which is a huge issue in onshore fish farming. “If we move it offshore, then there will be continuous tidal movements,” he said, “so the waste will be flushed away from the area, with continuous recirculation of waste.”

Whether more platforms can be converted into tourist attractions like Seaventures is an open question. Accommodations on the dive rig here are bare-bones: The rooms are 40-foot steel shipping containers with steel lockers for clothes. Upkeep of the building is expensive. The platform’s crew said that the seawater corroded its metal frame quickly and that repainting was required several times a year. Getting replacement parts for its elevator and generator is difficult, they said. “It’s really just hard-core divers who stay with us,” Ms. Harris conceded.

The larger debate over how best to retire aging platforms isn’t going away. In the North Sea, off the coast of Britain, for example, roughly 470,000 tons of offshore assets — including platforms, pipelines and wells — will need to be taken offline and recovered by 2022, at an estimated cost of more than $16 billion. The British government has offered energy companies and others large tax breaks to help them weather the expense.

And as oil and gas companies venture farther out to sea — and into more extreme settings, like the Arctic — to tap fossil fuels, the disposal challenges are bound to get more complicated.