The U.S. Court of Appeals here ruled yesterday that a Foreign Service officer did not violate State Department travel regulations when he took his family on a 12-day, $12,000 Mississippi riverboat ramble as part of their return trip from an overseas assignment.

Writing the majority opinion for a three-judge panel, Judge Laurence H. Silberman said the Foreign Service Grievance Board had not abused its discretion when it said the government must pick up the bill, and therefore the court should defer to the board's decision.

But the court's best-known advocate of judicial restraint, Supreme Court nominee Judge Robert H. Bork, disagreed, saying that the diplomat was simply up the river without a paddle.

The case involved the return trip of a United States Information Agency employe, Christopher Paddack, and his family from a tour in Uruguay and the State Department's regulations that require

air travel in one direction but allow different modes of travel, including surface transportation, in the other.

Paddack, now a creative arts programming officer with USIA here, received approval from departmental travel officials before he took his family on the 1982 journey that began with a flight from Montevideo to Santiago, Chile; continued with a ship to Lima; a flight to New Orleans via Miami and a cruise on the Mississippi Queen from New Orleans to his destination of Burlington, Iowa.

The State Department did not question the trip until a USIA employe put in a revised travel voucher for Paddack requesting per diem for the 12-day trip.

The expensive journey later attracted the attention of the General Accounting Office. Ultimately, the matter was referred to the grievance board, which upheld the expenses.

The government later asked a federal court to overturn the board's decision.

U.S. District Court Judge Thomas A. Flannery did overrule the board, saying that Paddack had not followed the State Department's overall instruction that travelers should exercise "good judgment" in all their arrangements, spending the government's money as if it were their own.

"This was, given the deferential statement of review applicable here, a determination the court was not free to make," Silberman said. He was joined by Chief Judge Patricia M. Wald.

"Riverboats may be a 'normal mode of transportation on the river,' " Bork said, "but riverboats that are luxury hotels and provide tours of historic sites at stopping places are not."

Walter H. Fleischer, Paddack's attorney, said yesterday, "We are grateful that the busy court troubled to review the facts and the law with great care and correct an injustice against" the USIA employe.