I have not been paid nor am I associated to any of the products outlined in this guide. These are simply the ones that I use. There are many alternatives than you can or may already be using.
I chose bulb mode as I wanted a long capture for movement in the water, but you can choose shorter shutter speeds, for different subjects.
In bulb mode, just click to take photo and click again to stop capture. I selected 16 seconds. As automatic mode is selected exposure is spot on and captured in camera.
Once the photo is captured, and you are happy, you can save to camera roll. N.B. the app has 3 settings to save, make sure you have selected large, which is available for iPhone 4, 4s & 5.