Great topic!
I'd certainly agree that wildlife filming has its unpredictable aspects, and that one can never know exactly what will happen. And if most of your movie-making is simply to record your vacations for fun, then only a minimum of planning might be warranted.
If you aspire to creating movies for some other purpose, a considerable amount of pre-production work can be rewarding. What are you setting out to do? If it's more than just having a good time, examine your goals carefully and what steps you need to get there.
As SimonSpear points out, time and money are issues. But you don't have to be limited by how much time and money you have. Think instead of how to accomplish your goals -- if you don't have enough money to make your movie, how can you get the $$? Who else would want that movie to be made? What's it worth to them? If you don't have the time for research and visiting the same spot over and over, how can you make the time? Who might want to help you get done what needs to be done?
You can spend 9 hours in the water "hoping" to catch fish mating, then feel defeated that you didn't succeed. Or you can spend 9 hours researching the fish's behavior and learn that your "watch window" is a certain size and organize a bunch of divers to be in the water with cameras all throughout that window. Just depends on how important it is to you. If you have a story inside of you that just needs to be turned into a movie, you can move heaven and earth to do so. And if you're going to be maxing out your credit cards and taking out a 2nd mortgage on your house, a lot of pre-production, including a business plan, is highly recommended! I also recommend making some allies you can trust!
Of course, you need to factor in the fact that you're filming wildlife -- you can create a story, a script, a storyboard and all that, as long as you do it with the understanding that there is more than one way to portray things. If you can't get the shot you want, your story doesn't have to go into the crapper, just figure out another way to say the same thing on film.
FWIW, personally I'd rather see more visual stories about the amazing things going on underwater and fewer "this was my really cool dive" videos.