Attaching a dive flag to your body is generally not a good idea. If you need to attach a line from yourself to a dive float, use a lanyard with a quick-release buckle. That way if the float were to get snagged by a passing boat you could release the line and prevent getting dragged upwards. When spearfishing, many spearos prefer to attach a line from their float to the butt or handle of their speargun. That way, if you happen to run into trouble, you can release your speargun and swim unencumbered to the surface.