ferry tickets for rebun rishiri

hi! we are travelling to rebun and rishiri in end May 2023.

we will be taking the ferry from wakkanai to rebun, rebun to rishiri, and then rishiri back to wakkanai.

is it risky to buy ferry tickets on the day itself, instead of booking in advance? we are looking to buy economy tickets and saw that if it's full we won't be able to board. however, we also understand May is non-peak.

thanks!

We visited the islands twice, and on both occasions we bought the tickets on the day, an hour before the departure, no issue.

Thanks so much. Thats very assuring :)