I came to realize that a lot of things in life are easy when you practice beforehand. It's a good idea to identify things you don't expect how to handle and then go and learn to handle them. It's especially important to learn how to use the tools you may need in emergency so that they won't surprise you when you need to use them quickly. Otherwise having those tools only gives you false sense of confidence, which may be dangerous.
So for instance, I bought a fire extinguisher and used it up in some deserted bushes. It was a small expense, but now I'm fully aware how this particular type of extinguishers behave. Or when a friend wanted to buy mace (she was coming home late at night), we bought two and used up one for doing target practice, so that she's aware how the fluid stream behaves and how to aim it.
The biggest problem with this approach that I find is that there are so many things you could prepare yourself. Even if it usually would just take a few minutes and little to no expense, you can't train for things you don't realize exist as concepts or situations. For instance, thanks to this subthread it is the first time it occured to me that a situation like obstructed surface when diving in a pool can occur. I'm adding it to my mental list of things to prepare for when I get the opportunity.
Having done whitewater kayaking for a long time I cannot stress this enough. You need to practice being under water with being denied surfacing even if you strongly want to.
Once you are calm, you can orient yourself by observing air bubbles for example, even in turbulent water.
Like people who do kayaking practice how to get out when the kayak turns upside down.