As important as the planning of the needed scenes the post production is very important. These are all the activities where you are composing your scenes to one story.
This can best be done with a powerful PC or a graphic workstation with a cutting software. The higher the quality/resolution of your raw material (SD, HD, UltraHD) the higher are the performance requirements to that Hardware.
You might have to pay special attention to the lights and color corrections when filming underwater. To solve that you have either to possibility to do that during the production (filters, camera settings) or you can adjust this during post production (color correction and color grading). Again – when light is not optimal and you have saltwater it appears blue and fresh water appeas green.
Keep in mind – also with the best lights available, when diving below 5 meters and filming objects or landscapes which have a distance of more than 2-5 meters the above mentioned situation is relevant.
With some knowledge in the theory of colors you will be able to do a software correction. But in general: if the raw material is bad e.g. completely dark or too much light you will not be able to do anything in post.
What can also be corrected are slight shaking scenes and color- and lens distortions.
At the end all the different scenes will be composed to a story, some sub titles will be added if necessary, an intro and an outro can be designed, background music will be added, a speaker text can be recorded and applied to the video and also background noises (atmo).