rickg , a few posts up, makes some good points. They are nuts for cell phones there in Indonesia.
I'm not sure how practical cell phones are though, to be honest.
There are few waterproof models for a start, which makes using them in the ocean a pain in the backside, especially in the choppy conditions you might expect to get lost in. Then you need the right SIM card for the country, even assuming the cell phones are compatible (GSM vs analogue vs 3G)... then you need to keep it charged. Finally you need to be in range of a cell site which may not always be the case in these remote places.
Even if you do get through, you can say "I'm floating in the ocean"... it is much easier to direction find on a VHF source rather than have to go through the cell phone provider to DF on the cellsite, which generally requires overlapping cells anyway (if I remember correctly from my communication engineering days!)
I think I would prefer a marine radio for my survivial tool, and make sure both the boat and the diveshop land staff knew the transmit frequency - maybe leave a printed sheet with the details with the disclosure forms you sign for most dive shops.