There is no way to prevent the explosion at the moment without using mods, but there are ways to prevent or mitigate damage from the explosion. For those who like to use TNT, you will have found that it has little effect while under water. With this knowledge in mind, dumping a bucket of water on the creeper at the right time can cause the explosion to be absorbed, but it requires you to be very precise with your timing.
Stop Creepers Exploding
Currently, the best way to prevent a creeper from exploding is to kill it before it has time. The creeper will only explode when it has been standing next to you for long enough. If you can run backward and attack it as you do, it will prevent it from exploding as quickly and give you time to kill it before it has the chance to do any damage to you. Easier said than done of course, when you are inside a building it limits the room to run. This is why it is always useful to have a good sword at hand to do good damage.