Entering and getting visa question

I bought a ferry ticket from Busan to Osaka (one way). I'm from the USA and have a question about getting a visa. I have my itinerary that shows I am going to be leaving Korea on August 16th, but I don't have a ticket (ferry or otherwise) actually out of Japan because I was going to stay about 3 weeks (maybe more or maybe less).

Do you think this will be a problem? Obviously, if I am going to be flying home in August, I must be leaving Japan by that time. Any help would be greatly appreciated!

No, you will have no problem entering Japan via international ferry.

without a visa and a return ticket out of Japan, you will probably denied entry.

a return ticket out of Korea does not count as two countries do not share a common immigration procedure.

Japan faces a high number of visitors with overstay violations, with random checks to verify visitor has proper ID, and legal right to stay--and it stepped up its Immigration restrictions.

Advised to have a prepaid return ticket out of Japan.

A return ticket out of Japan would definitively show that you plan on leaving, however I am also of the opinion that you will have no trouble entering the country without one. While they can deny you entry for not having one, Japanese immigration tends not to be very strict with Americans entering Japan and will probably accept your return korean flight as proof that you will eventually be leaving.