The homepage is in Japanese http://www.aioso.com/ and the only way to contact them to make a reservation is by telephone or fax (from within japan freedial 0120-84-2202, from outside of Japan TEL:083-984-2201 FAX:083-984-2279). I recommend faxing them.
Be aware that some rooms do not face the sea, so make sure you say you want a room on the sea side ("umigawa") of the hotel.
The do not have many rooms so they get very booked up, but they accept reservations for up to six months in advance.
Generally one pays for evening meal, breakfast and use of the bathes. Prices start at about 80USD (8000yen) per person per night depending upon food. They also offer just a room and bath for about 5000 yen on weekday nights outside of the main holidays, but they may not be pleased to offer this option unless in an off season. I recommend the cheapest "Normal Accomodation Plan C" as detailed in the photo here http://www.flickr.com/photos/nihonbun...
I phoned them. This was the conversation Lady: "Kaibou No Yado, Aisou desu" (This is the Seaview Inn, Aiosou") Me: "(very slowly, in UK RP) I am phoning to ask whether you accept reservations in English" Lady: "Moshi Moush" (Hello?) Me: (even more slowly) "Hello. I am phoning to ask, do you accept reservations in English" Lady: silence, and giggling (#@$%!) Me: "Hello. Can you hear me?" Lady:"Moushi Moushi" (Hello) Me: "Please can I reserve in English?" Lady: "Not speak English." Me: "Thank you." (I hung up)
Providing no English, either online or in person, and no email address on a hotel homepage may seem rather unusual business practice. This may be because Aiosou is funded by the local council.
Come to think of it, if one phoned an Australian or even Hawaian Hotel and spoke in Japanese, then despite the large number of Japanese visiting such establishments, one would be unlikely to get served. In fact, if one spoke in Japanese then people might just put the phone down on you.