Continuing strength in its hotel and food operations gave Marriott Corp. a 6 precent gain in third-quarter profits, while another Bethesda-based company, Survival Technology Inc., posted a 15 percent decline for the year.

Marriott earned $29.9 million ($1.10 a share) compared with $28.2 million ($1.05) last year. Third-quarter sales jumped 25 percent from last year, to $631.7 million from $503.4 million, largely because of expanded hotel and restaurant operations.

"For many of our operations, 1982 marked the most difficult business environment in many years," said President J.W. (Bill) Marriott Jr. "The problems of the airline industry and the impact of the recession on business and tourist travel in recent months are well-known. Overall, our managers have performed well under severe conditions. Such performance should yield strong benefits once the economy turns around."

Marriott's airline food service and amusement park divisions showed declining profits for the third quarter, but increased earnings by hotels, contract food services and restaurants offset the drain on the company's bottom line.

For the nine months ended Sept. 10, Marriott had profits of $69.2 million ($2.54) on sales of $1.7 billion, compared with earnings of $64.4 million ($2.40) on sales of $1.4 billion last year.

At Survival Technology, research and development expenses cut into the $20.3 million in sales this year, a 13 percent increase over last year's $18 million. The company posted profits of $748,000 (29 cents) compared with $883,600 (35 cents) in 1981.

Fourth-quarter results showed an earnings increase, however, to $412,000 (16 cents) from $394,000 (16 cents) last year. Sales increased to $5.8 million from $5.2 million in the same periods.

Survival Technology manufactures medical devices and provides services for emergency health treatment and for the military.