Vessel Hotel is a popular place. Even in December, the place is almost full. Japanese Families took up at least half of the rooms. Hence if you do not like lots of children around, tough luck to you.
The room are large for Japan standard, The hotel is clean bright, service is good. And the included breakfast is a big plus. It is within walking distance to the city centre and the ferry terminal, and there are free car parking.
Ishigaki is sunny and warm in December, the sea breeze is lovely and it is truly an enjoyable time to visit. If you want to see the sights, hiring a car will be an excellent idea.