Key Biscayne, Fla. — The Jamaica that Edward Seaga inherited last year when he became prime minister was a nation lurching from crisis to crisis. Now, comparing Jamaica to a car careening out of control and going downhill, Mr. Seaga says ''We've stopped it and turned it around.'' He adds: ''What a mammoth achievement!''
Mr. Seaga and his associates in the Jamaica Labour Party are not entirely happy with their first year in office: Things have moved more slowly than they expected.
Basically the most important achievement has been putting Jamaica's financial house in order.
Mr. Seaga is convinced that the groundwork has been laid to get Jamaica back on its feet. He has been able to attract significant loans and new investments from abroad, including the US.
When Mr. Seaga was elected 13 months ago, the economy was in a shambles - and his opposition, incumbent Prime Minister Michael Manley, agreed with that description. Another year of negative economic growth seemed certain; supermarket shelves were bare; foreign reserve coffers were almost empty; violence raged in the streets of Kingston, the capital; and tourism, a major money-earner, had fallen off drastically.
During his second year in in office Mr. Seaga hopes to get that car moving uphill again and return Jamaica to ''the sort of economic growth that was evident in the 1960s.''
That was before Mr. Manley and his People's National Party came to office.
Even if some of that takeoff has not taken place during 1981, Mr. Seaga is extremely pleased with the achievements:
* Gross national product, instead of falling during the year, will probably grow by about 1.5 percent - not the hoped-for 3 percent, but certainly better than the rate of the past three years.
* The government has obtained about $1.5 billion in loans and credits - a $ 698 million loan agreement with the International Monetary Fund, which had suspended credits to the Manley government, and some $900 million from other lenders, most of them in the US.
* Inflation, which was running at 18 percent when Mr. Seaga took over, has averaged about 6 percent during his first year; last month the rate fell to a 4 percent annual rate.
* Although prices are high, groceries are back on the supermarket shelves. People no longer stand in line to purchase many of the items in short supply.
* Tourism, which had dropped so dramatically in 1979 and 1980, is now back up to its 1978 level, bringing in some $200 million in 1981.
Jamaicans are counting on a 70 percent hotel occupancy rate this winter season, which began Dec. 15. Last year the rate was a mere 45 percent.
To attract more tourists, particularly from the US and Canada, the Seaga government last month began a $7 million advertising campaign entitled ''Make it Jamaica Again.''
Investments in orchids, roses, and papayas are paying off; exports of these crops are up dramatically.
But the Seaga government is faced with some staggering problems that even the prime minister admits will ''defy solution.'' There continues to be a large shortage of foreign exchange, and unemployment is extremely high. The official rate is 26 percent, but Jamaican officials say privately that unemployment is closer to 35 percent. One suggested that in certain ''enclaves'' it could be well over 50 percent.
Mr. Seaga said that he hopes to cut unemployment with such programs as one for rural road repair.
But, he admits, ''We don't expect to cut back on unemployment in a large scale until 1983,'' as the effect of new foreign investment is felt.
Mr. Manley has refrained from criticizing his successor during his first year in office, but there is now evidence that he is getting ready to attack Mr. Seaga's policies.
During the campaign Mr. Seaga promised ''deliverance'' from Jamaica's economic woes. Mr. Manley said last month, ''Deliverance has indeed come and it has failed completely. Now we are ready to work for redemption.''