United Nations, N.Y. — The final act of the Law of the Sea conference will be played in Jamaica on Dec. 10 when many - but not all - of the world's nations sign a treaty to regulate the uses and resources of the oceans.
This international compact, which took almost nine years to negotiate with the participation of 155 nations, affects almost all aspects of maritime life - commercial, economic, legal, and military. It gives ships the right to pass through important straits and waterways. It allows each country an economic zone of 200 miles and a territorial zone of 12 miles. It settles questions regarding fishing rights and continental shelves. It spells out rules for seabed mining and establishes an International Seabed Authority (ISA) to govern the harvesting of ocean minerals.
''The treaty is a hallmark,'' says former Canadian Secretary of State Mark MacGuigan. ''It is the most important legal document since the adoption of the UN Charter.''
More than 60 states are expected to sign the treaty, which was adopted last April in New York by a 130-to-4 vote, with 17 abstentions. The signing ceremony means a point of no return for the treaty. It will from then on become the international law of the seas. Altogether 145 countries are expected to sign it, some within a few months, others within a year or two. The United States, Turkey , Venezuela, and Israel alone still reject the treaty; and a handful of others, mainly Latin American, remain hesitant.
The Scandinavian nations and most of the communist countries that abstained last April have let it be known they will sign it. So will France, Japan, Canada , Australia, Brazil, Mexico, Norway, and Ireland. West Germany and Britain will not sign at this time, mainly to save the US the embarrassment of being isolated. Both countries are expected, however, to eventually sign the treaty.
The ISA, with headquarters in Jamaica, will set up its own mining enterprise, retaining some mine sites and awarding others to private companies. Those firms, however, would be required to sell their technology to the global authority under certain circumstances and to abide by production ceilings.
The Reagan administration opposed ISA provisions which it considered harmful to US interests and contrary to its philosophy of free enterprise. The US had been the instigator of the Law of the Sea Treaty, which encompasses much of everything that the US originally expected it would.
''This turnabout, along with US turnabouts regarding international trade (such as wheat sales to the Soviet Union and sanctions against US-related companies in Europe fulfilling contracts for the Soviet pipeline) damages the country's international reputation,'' a leading West European banker says.
The treaty covers navigational rights for warships to sail through straits, overflight rights for aircraft, 200-mile economic zones for nations, and boundaries for the continental shelf.
''The US now makes two contradictory claims. On the one hand it argues that all that is in the treaty, except the seabed mining provisions, is already customary law and that it can profit by it without signing the treaty. This is sheer nonsense, since the treaty indeed creates a large amount of new laws. ''On the other hand the US says that since it will not sign the treaty, it will not be bound by it. The truth of the matter is that inasmuch as the treaty is becoming customary law, all nations are bound by it, whether they have signed it or not,'' a legal expert says.
Meanwhile, US companies that are eager to exploit the mineral-rich (manganese , cobalt, nickel, copper) ocean floors will lack legal protection for their investments and thus may not risk mining in international waters. Banks are not expected to finance unilateral moves in this connection, since they may lead to litigation and be brought to the International Court of Justice.
The administration's attempts to persuade other industrial nations not to sign the treaty and instead to sign a mini-treaty with the US - which in fact would allow the rich nations to ''go it alone'' - have collapsed. In a last-minute attempt to turn some major Western partners around, the US State Department recently sent high-level representatives to various allied capitals. They were listened to politely but reportedly failed in their mission.
The authority's preparatory commission, which begins its work in March, will draft detailed mining regulations. Nations that intend eventually to engage in seabed mining are expected to seek a seat on the commission in order to have a say in its decisions. Only countries that have signed the treaty can occupy a chair on the commission, another reason Japan, France, Canada and others have decided to sign it.
As a treaty expert points out, ''As soon as the authority opens, it must register pioneer investors, choose mining sites, etc. Decisions by the authority will be made by consensus, which means that they will be irrevocable.''
Singapore's Tommy Koh, the conference's president, who is credited with saving the negotiations from wreckage several times, says he ''deeply regrets that the US suddenly reversed course last year.'' He says it is a good treaty, ''the best obtainable,'' and that in a few years the US will sign it rather than remain secluded in ''proud isolation.''