Navy officials said the lobster boat MN Wayward Wind turned over in a giant wave with 50 people aboard about 35 nautical miles (65 kilometers) southeast of Puerto Cabezas.
Some of the survivors used canoes from the fishing boat to stay afloat, while others were rescued by a sailboat, the navy said.
The experience of the crew has given authorities hope that the 18 still missing will be found alive, Rear Adm Marvin Corrales said.