Sightseeing cruises along the Potomac River between Washington's waterfront and Mt. Vernon will resume next spring under new management.

Washington Boat Lines Inc., which purchased assets of the failing Wilson Boat Lines in September, announced that regular daily cruises and special charter trips will begin on March 31.

The company's decision marks a new beginning for the old Wilson line, operated from the waterfront continuously from 1876 until last fall.

According to Thaddeus Lindner, chairman of Washington Boat Lines, his firm has acquired a 353-passenger vessel, the Diplomat, now anchored at Pier 4 on Water Street SW.

The twin-decked, 125-foot boat actually is returning here after four years in the Boston harbour under the name Bostonian. Previously, it had been one of the old Wilson Line boats for a decade.

Operating head of the new service will be Willem Polak, who has operated the Potomac Boat Tours here for the last three years.Boat Tours will continue to operate its boat, the Spirit of '76, between the Lincoln Memorial and Georgetown, but one office will handle reservations for both.

Washington Boat Lines spokeswoman Midge Lippman said yesterday that fares next summer will be the same as during 1978 on the old Wilson line: $5.50 round trip for adults and $2.20 round trip for children under 12 years of age. Admission to Mt. Vernon is $3 for adults and $1 for children. CAPTION: Picture, The 353-passenger Diplomat, which has been acquired by Washington Boat Lines, at Pier 4 on Water Street SW.By Margaret Thomas-The Washington Post