Ferry Service from Shimabara to Kumamoto

We plan to take the ferry on our rental car from Shimabara to Kumamoto on Friday morning. Do I need to make advance reservations with the Kyoshu ferry service or can I just turn up at Shimabara port on the scheduled departure at 11am. We are a family of 5. Would anyone recommend visitng mt Unzen on our way from Nagasaki to Shimabara ? we also plan to visit Mt Aso in Kumamoto prefecture.

With a car it is better to book ahead for the ferry. We did that from Kumamoto to Shimabara during mid March and the ferry was unexpectedly quite full. We asked our hotel in Kumamoto to call ahead and do the reservation for us.

Unzen is worth a visit even without staying overnight. If not taking the ropeway and going for a hike in the surrounding area, the tour of Daikyokan Jigoku hell area in town would take less than an hour to walk around.

Time permitting, Shimabara is also worth visiting for the castle, carp-filled streams of the samurai district, Mt. Unzan Disaster Memorial Hall and the Suffer House Preservation Park/Sembongi District.