For over a decade I've entered Japan on one way tickets and no airline has asked for my proof of return/onward flights, so I stopped obtaining them long ago.
However a few days ago I arrived at
BKK airport and VietJet Air would not allow me to board my flight to Fukuoka without a ticket out of Japan!
I arrived three hours early but ended up being the last person at the check-in desk trying to buy a ticket out of Japan, which I hate doing without lots of time to weigh everything up. Since there are 3rd party ferry booking sites (such as "Direct Ferries") which claim to cover all ferry trips but don't seem to always have accurate/up-to-date info, it's very frustrating trying to find the true sites for the ferry companies themselves.
I believe there are currently two companies or at least two routes though there used to be at least three. There is a Shimonoseki-Busan route with an overnight ferry and a Fukuoka-Busan route with a fast hydrofoil. I'm not certain whether the same company runs both or if there are currently any others. I could only find one and it told me that at the current time (presumably due to getting going again after a long covid outage) that booking is only possible by telephone for now.
I gave up under pressure and bought a flight to Taipei from Okinawa but am still wondering if I could've persisted and booked a ferry that I just failed to find the right website for. Is it possible?