The Long Island garbage barge plying East Coast waters in an unsuccessful search for a disposal site was off south Florida and headed back to New York. A Coast Guard spokesman said that the scow, with its 6 million pounds of trash, by the middle of the week could be back at Islip, N.Y., where the 49-day odyssey started. Meanwhile, Alaska Gov. Steve Cowper responded negatively to a letter sent to 48 states seeking a home for Islip's solid wastes. "We're interested in the import-export business," Cowper said, "but this is not exactly what we had in mind."