As much as possible prepare the settings on your camera before you move in to take the photo. Assess the light conditions and distance to subject then if you are working in Manual mode adjustment the shutter speed and aperture. If you are using strobe/s adjust their positions. If you are not using a flash then adjust white balance. If you want to take a macro shot select that setting.
By doing this once you are in position and have a shot composed you only need to make minor adjustments to your camera settings and light source. You can concentrate on getting the subject in focus and composing the photo.
Each time you take a photo check to see if it is correctly exposed and make further adjustments if need be. It is possible to correct images with computer software but getting in right in the water is better. 😉