Garuda is the national airline of Indonesia with typically newer aircraft and less restrictions on their luggage allowance. All the other airlines are privately owned: Merpati, Lion, Batavia, Express Air and one or two no-name brands. These airlines make their money on excess baggage, and they LOVE the tourists with their cameras and dive-gear.
Be careful; allowances change weekly, some airlines will charge extra after 15Kg and others after 20Kg. Try and carry your heavy stuff on board:- batteries, lenses, toiletries etc. They don't seem to care too much about carry-on luggage. Just remember to be ready when boarding commences, shove and push your way to the door, otherwise there will be NO space left in the overhead lockers when you get to your seat!
You can save at least 5Kg on a soft bag instead of a heavy dive-bag; just wrap your dive-gear in bubble-wrap and pack it tight! You can also hand-carry your housing with the camera inside!
Just make sure you carry at least 2 million Rupiah ($200) on you, (ATM's are EVERYWHERE in even the smallest towns!) and most of the time the airline guy at check-in is happy to make a 'deal'.
Never lose your cool, be humble, friendly, and patient; it will take you long way (and save you some money!)