Boat Kagoshima-Yakushima

Hello everybody,
I plan on visiting Yakushima Island from the 1st to the 6th of January.
I have found lots of useful information on japan-guide, but nothing about how to book a boat between Kagoshima and Yakushima. It appears it can be done on the websites, or by calling them, but absolutely everything booking-related is in Japanese and I do not speak a word. I tried to send emails to the boat companies, but never got any answer.
Would somebody happen to know:
- If I can buy tickets to any of the boats (quick ones and slow ones) just before departing?
- If yes, is it a peak season or will it be fully booked and leave me stuck?
- If no, how could I book a ticket beforehand?

Thanks a lot for anybody reading this, and for answering for the ones who know a bit about this!

If I can buy tickets to any of the boats (quick ones and slow ones) just before departing?

Yes, if the boat is not booked out, then you can buy it at the terminal just before departure.

If yes, is it a peak season or will it be fully booked and leave me stuck?

It is a peak holiday season, but I am not familiar with the traffic pattern for Yakushima over the New Year holidays.

Alright, thanks for your answer. Then I will try to book beforehand, but if I cannot manage, I will make sure I arrive early enough on the day of departure to buy one of the last few tickets hopefully ;)