Dan_T would know better than I would, but I found IDR 100,000 notes to be convenient units of currency for tipping dive guides, drivers, etc. At current exchange rates, that's about USD 7.50, but it's a lot to someone whose wages might otherwise be, say, 600,000 in a week. Carry a wallet full of 100,000s and you feel like a millionaire. In another thread, in which I inquired about tipping before my (land-based) trip, the consensus seemed to be that IDR 100,000 for a dive guide per day for a diver or diving couple was fairly normal. For a liveaboard, where you are getting a lot more service than just a guided dive, and where will be leaving a relatively big tip all at once, maybe US dollars are preferable for the sake of convenience. Again, my recent trip was land-based, and I have no idea about liveaboard tipping.