A new ferry service from Manhattan to the Mets' Shea Stadium in Queens received the Mayor's blessing this week. Scheduled to start March 31, NY Waterway's "Mets Express" will carry baseball fans from South Street Seaport to the World's Fair Marina in Flushing Meadows Park, a five minute walk from Shea Stadium. Several Queens groups, however, have objected to the service, citing concerns about the Flushing Bay ecosystem and the use of parkland for a commercial enterprise.