Joseph Curran, founder and first president of the National Maritime Union, died of cancer Friday at Boca Raton Community Hospital. He was 75.

Mr. Curran, who founded the union in 1937 as a unit of the Congress of Industrial Organizations (CIO), was credited with leading the nation's major union of deckhands through its turbulent first three decades. Known both affectionately and apprehensively as "Big Joe," he commanded allegiance or hatred from his 55,000 members --there was no middle ground.

Described by friends as a loyal, intelligent, self-educated and honest man, he helped make American merchant seamen the best paid and fed in the world. His admirers also pointed to the union hiring halls he set up to end a corrupt shape-up system of employing seamen and to the fact he broke the color line aboard American ships, opening the ranks to minorities in 1943.

Mr. Curran's critics, however, regarded him as a man who forgot the rank and file, put his own interests first and treated the union as his personal property. They blamed him for jobs vanishing and pensions being threatened when shipping interests fled to foreign flags.

He was accused of expediency and dictatorial highhandedness, particularly on the issue of communism. Enemies said he allowed communist infiltration of the union after World War II when it helped strengthen his control, but turned against the Communists when it was to his advantage. He personally led strong-arm squads in street brawls before purging Communists from positions of influence in 1948.

Firmly entrenched, Mr. Curran ran his salary up to $85,000 a year by 1972 and enjoyed virtually unlimited expense accounts. The union's Manhattan headquarters building, featuring huge portholes instead of windows, bears his name.

A native and longtime resident of New York City, he began his stormy unionizing career in 1936, when he organized a job action aboard the S.S. California to show sympathy with seamen striking the ship. He organized the NMU after leading a major strike against ship owners.

In 1973, Mr. Curran retired in the midst of a battle with dissidents who sought to block a $1 million lump-sum retirement benefit. A recent Federal Court of Appeals decision found pensions of NMU officials to be not unreasonable.

To honor Mr. Curran, the union plans to close the nation's ports at noon Monday for several hours to allow seamen to attend memorial services.

Survivors include his second wife, Florence, of Boca Raton, whom he married in 1965, a son and daughter, and two grandchildren.