It has been my observation that people who can’t swim (adult or child) often have difficulty putting their face in the water. With kids, for example, if you are very careful not to get any water on their faces (other than a damp washcloth) while giving a bath, they don’t get used to getting water in their face. It doesn’t matter if it’s from a squirt gun or a pool - it makes them panic. All of the non-swimming adults I’ve asked had a parent who didn’t let the slightest drop of water near their face in the tub and these adults continue to feel uncomfortable with water in their face.
If this is something that makes you nervous, you can practice in the tub or the shallow end of a pool. Or, you can start with a soaking wet washcloth on your face for a few seconds, leaving it on a bit longer and longer each time. Then work up to partially submerging your face (up to the nose for example) in the shallow end of the pool. Eventually, you can work up to putting your entire face underwater. And it’s ok to hold your nose - we’re not talking winning a gold medal here.
I know this sounds stupid but feeling comfortable with putting their face in water really can be an issue for people who don’t know how to swim. I’ve taught a lot of friends (and their kids) to swim by slowly getting them used to getting their face wet. Once they get the hang of that the rest is far less intimidating.