Carnival Corp may stick to its plan to sail in US waters by December, thanks to a federal judge's change of heart.

US District Judge Patricia Seitz on Friday indicated that Arnold Donald-led Carnival had to give 60 days' voyage notice before she cleared a ship as environmentally compliant.

However, she decided in Wednesday's final ruling — given along with probation and a $20m environmental fine handed down months ago — that it had to give only 30 days' notice.