I've travelled for Thanksgiving and dining areas and public areas of the ship were decorated for the Holiday. Featured meal was traditional turkey with all the trimmings but there were any number of choices as well.
My husband and I went on a cruise during last Thanksgiving and YES, they did offer your traditional turkey meal on the menu but to be honest, NO ONE at our table ordered it. We all wanted lobster & some of the other wonderful choices. But they did offer it.