I once went swimming at a YMCA pool with my brother when I was around 14 or so. I had only practiced in a smaller pool so I was pretty excited to splash around in a really large one! My brother suggested that we race across the pool to see who's the fastest, but what he forgot to mention was that instead of slowly getting deeper as you went down the pool, it has a shallow area and then drops into a super deep part.
So the race began and I was in the lead (don't worry Iwaku, your boy Specugami didn't let ya down >:D ). I had crossed into the deep end and my brother called out for me to turn around, but I didn't hear everything he was saying, so I stopped to listen and that's when I started to sink. It was unexpected, so of course, my first move was to thrash around in a panic :"D haha. I just kept sinking deeper and deeper and eventually, I couldn't hold my breath anymore. I barely remember being saved by the lifeguard or anything after that. As far as I really recall, I was drowning one moment and the next I was out of the pool and being checked on by a nurse on staff while my older brother freaked out.
Surprisingly, I'm not afraid of water. I mean, I'm wary when it comes to large bodies of water but not enough to avoid them. I'm scared enough that I try to avoid water deep enough that I can't touch my toes to the bottom (good thing I'm 6'5!).