There was talk of an investigation going on in Canada that coincided with the arrests of these men. I don't know how credible the report is considering where it comes from, but here is what ABC News was reporting on their website a couple of days ago:
Law enforcement officials say they are not sure exactly what "watermelon" referred to, but they think it may have been a code word for some kind of terrorist activity. The arrests of the six men on Friday, Saturday and Sunday came as agents investigated reports of a possible "dirty bomb" being moved across the border from Canada, sources said.