July 4th Celebrations, on a river cruise for the night, what should I wear as a British tourist, wanting to join in the celebrations, but not cause offense.

Really, as the title asks. We will be on the Horizons Edge river boat cruise as British tourists wanting to enjoy July 4th, and want to get into the spirit, but I'm worried about "dressing over the top" and offending etc. And when I say over the top, I mean, like wearing an American Flag tie or something like that. Not like dressing up as any kind of parody.