Marriott Corp. -- owner of a national chain of hotels, motels, restaurants and other food services -- reported increased earnings yesterday for the second quarter and for the first half of its fiscal year ended June 13.

Marriott reported net income for the quarter of $17.2 million (65 cents a share) compared with $17.0 million (45 cents) a year earlier. Revenues during the period were $389.2 million compared with $353.3 million.

First-half net income was $29.9 million ($1.03) compared with $28.8 million (76 cents). Sales were $740 million compared with $656 million.

The gain in the earnings per share outpaced the increase in net income mainly because of the company's repurchase of about 12.5 million shares of its common stock since the first quarter of last year, the company said.

The company's operating income improved 12 percent during the second quarter because of continued strong performance in the company's hotel group and U.S. in-flight airline catering service, the company said.

Meanwhile, the company announced it will manage a 395-room hotel under construction on Boston's Long Wharf. The project is scheduled to be completed in early 1982. The building will be designed to complement the historic mercantile buildings along the wharf, the company said.

The hotel also will have a 155-seat restaurant, a health club, indoor pool, separate saunas for men and women, an exercise room and game room.

Computer Network Corp. of Washington reported decreased earnings for the year ended March 31. Net income was $930,000 (55 cents a share) compared with $1.5 million (90 cents) a year earlier. Revenues for the year were up slightly, from $21.0 million to $21.2 million.

Fourth-quarter earnings were also down, from $583,000 (35 cents) to $35,000 (2 cents). Revenues for the quarter were $4.9 million compared with $5.7 million.

STSC Inc. of Bethesda, formerly Scientific Time Sharing Corp., yesterday reported record revenues and net income for the year ended May 31. Revenues for the year were $21.1 million, a 27 percent increase from $16.6 million in 1979. Net income was $1.5 million (86 cents a share), up 51 percent from $980 million (61 cents).

Fourth-quarter revenues were $6 million, up 30 percent from $4.6 million, and net income was $411,000 (24 cents) compared with $293,000 (17 cents).

Baltimore Gas & Electric reported a drop in net income for the five months ended May 31 from $56.6 million ($1.58 a share) to $46.2 million ($1.16). Revenues were $489 million compared with $447 million.

For the 12 months ended May 31, the utility reported net income of $116 million ($2.98) compared with $121 million ($3.31). Revenues for that period were $1.1 billion compared with $1.0 billion.