The upstairs room at an old plantation house on the Caribbean island of Curacao was insufferably hot. But jazz vocalist Gregory Porter was cool and collected, even while wearing his trademark winter cap. Known for his silky smooth voice and socially conscious lyrics, Porter has reason to be confident. His first album, “Water,” garnered critical acclaim and a Grammy nomination for the 2010 Best Vocal Jazz album. This February, he released his second album, “Be Good.” And now, he’s in the Caribbean to perform at the annual Curacao Jazz Festival.