The United States and Cuba yesterday announced the signing of maritime and fisheries agreements, and the chief U.S. negotiator forecast "constant improvement" ahead in Washington-Havana relations.

Assistant Secretary of State for Inter-American Affairs Terence Todman, just back from the first visit to Havana by a U.S. diplomat in 16 years, disclosed that he had discussed a broad range of issues with Cuban Foreign Minister Isidoro Malmierca.

The talks indicate that "the chances are good" for a gradual, steady improvement of relations on a step-by-step basis. Todman said.

Cuba is likely to take some positive steps of its own, which he would not specify, to further the drive for closer ties, Todman said.

On the U.S. side, the State Department is considering a plan to station American diplomats in Havana as a "special interest section" of the Swiss embassy, which handles U.S. affairs there. This is the probable next step in Washington-Havana relations, according to informed sources.

Cuba would be permitted to station its diplomats in Washington as a special section of the Czechoslovak embassy as part of the arrangement.

Closer ties with 14 nations which have no U.S. diplomatic relations is a general policy of the Carter administration. Another set of talks along these lines is scheduled to begin Tuesday in Paris between the United States and Vietnam. Senior officials of the two countries yesterday expressed optimism about the outcome. relations with Cuba on Jan. 3, 1961, four months before the abortive Bay of Pigs invasion by U.S. -backed forces, and the years since then have seen recurrent bitterness, invective and cross purposes.

Too many problems still remain to speak now about early "normalization" of relations, Todman said in his news conference yesterday. Nonetheless, his was the most optimistic official report in many months on the chances for improved ties, and it was the first indication that the Carter administration has begun to implement its plan to discuss the full range of pending issues with Havana's government.

Until now, the two governments had maintained that their recent contacts were limited to the largely technical issues of the maritime and fisheries agreements, which were signed in Havana late Wednesday and announced yesterday morning. Negotiations on these pacts began in New York a month ago.

The need for the agreements arose March 1, when both nations extended their jurisdiction over fishing rights 200 miles out to sea and the United States established strict limits on foreign fishing within this zone. Because Cuba is only 90 miles from Florida, the boundaries overlapped.

Details of the two agreements will be published later, officials said. However, the maritime pact is believed to involve a boundary midway between the two coasts. Under the fishing pact, which Todman said is "basically the same agreement" as with other nations. Cuban fisherman will be given detailed limits on the type and amounts of fish to be taken from U.S. waters.

A U.S.-Cuba anti-hijacking agreement, negotiated by third parties, was recently terminated by Cuba in protest of claimed U.S. terrorist acts. However, Todman said Cuba professes to be "firmly against hijacking" and that he presumes Havana will continue to honor the terms of the treaty.

President Carter said two weeks ago that the United States is seeking improved relations with Cuba on a "measured and reciprocal" basis. Todman cited those words and said, "From my conversations, the Cubans understand this."

Among the major issues from the Cuban viewpoint is the 15-year-old U.S. trade embargo. A week ago Carter reportedly told Sens. George Mc-Govern and James Abourezk, South Dakota Democrats who recently visited Havana, that he would not oppose congressional action to lift the embargo as it applies to food and medicine.

Carter has listed the presence of Cuba troops in Angola, Cuba's "aggravating influence" on other countries of Latin America and human rights in Cuba as barriers to normalization of relations from the U.S. viewpoint.

Todman cited U.S. claims for expropriated property, which total $1.8 billion, as another problem to be tackled. Some 50 companies and individuals with Cuban claims have recently organized a lobby group to argue that the claims be settled before the trade embargo is lifted.

On the fortcoming Vietnam negotiations. Vietnamese Premier Pham Van Dong said yesterday in Paris that "I am, I have been and I will always be optimistic" about a positive result.He said "all the questions" related to normalized relations with the United States will be covered in the talks to begin Tuesday.

Richard Holbrooke, assistant secretary of state for East Asian and Pacific affairs, told reporters here that "every argument" favors the normalization of relations if Vietnam continues to account for American missing in action. Holbrooke, who will head the U.S. negotiating team in Paris, expressed "genuine hope" for closer ties with Hanoi.