Seaborne Airlines and JetBlue are hooking up for some Caribbean codeshares that will let JetBlue sell tickets to destinations in the Virgin Islands, Netherlands Antilles, Martinique and the Dominican Republic.
Seaborne, originally based in the U.S. Virgins and now operating from San Juan, PR, serves 15 destinations in those areas with a fleet of 8 34-seat turbo-props and two 15-seat seaplanes. The codeshare agreement means that JetBlue will add its flight numbers to the flights, and passengers will have a single ticket, even though they'll change planes in San Juan.
Seaborne already codeshares with American, and with Spain's Air Europa, and has interline agreements with Delta and United. Interline agreements allow a carrier to sell tickets for the other airline's flight, but not as a combined ticket. But they do allow your bags to be checked through, not possible if you had to buy the two tickets separately.
Arrangements of both kinds are helpful both to a large carrier that can reach destinations not profitable for it to fly, and to the small carrier that can benefit from the passengers fed to it, and from reduced costs of advertising and selling tickets on their own.