Stayed here on a diving trip (probably the only reason you'd go to Aka Jima).The diving from the hotel was very efficiently and comprehensively organised (and our not speaking Japanese was NOT an issue) - all equipment is available for here, and three dives are possible during the day plus one at night if min 4 take up. Many dive sites available, and seemingly many dive instructors - all the ones we had were very professional. They will INSIST that you do a refresher dive if you have less than 30 under yr belt or not dived recently. These cost JPY 3,000 extra but you get yr own dive-master for that dive.
The hotel IS expensive, twice the rate on the main island for what is a very basic room - but rooms are few and far between on Aka Jima, and that's the way it is! All meals are available (and included), and all are Japanese style, good, freshly cooked in the open kitchen. Tasty and filling enough. My only surprise was that there was no fruit either at meal time or after the dives. There is a roof top jacuzzi / pool - but this was not in use when we were there. Bikes for hire - but strangely for the whole day only, so they lost a casual couple of hours' hire from us, whilst they stood idle - poor business practice. The beach is 15m from the hotel, but aside from that there is NOTHING to do on the island aside from dive trips. No smoking rooms - huzzah!!