Uzushio Kisen Curise is a great cruising tour to enjoy Naruto Spiral. From 8:00 to 16:30, every 30min. there is a tour cruises. The tour takes about 20 min. and travels to right under Onaruto Bridge where Naruto Spiral appears.
Naruto Spiral appears when a tidal current is happening. So, you can see the spiral around 2 hours when it is high or low tide. You can check out the time of the tide at the ticket office.
The best time to see the spiral is 2 hours between the tide.
Official Web: http://www.uzushio-kisen.com/index.html