Mine was also on a pool. Twice
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I was 6, was visiting a relative with my family, spending some days there, my aunt (the relative we were visiting) had just started working on a club and was allowed to live on a small house there, we spent some days there, some days on her sister house on the city, my mother have made very clear to me to not go to the pool if they weren't there, but one day when we were there, a sunday when the club was closed, nobody was there, and my older brother (two years older) decided to go the pool so I went with him, despite not liking to desobey my mother, he went to the adults pool, it was a pool that had a short size and a deep size, it went getting deeper the longer you went, my brother had enough height to stand on his feed on the smaller part, I didn't, he jumped it, teased me, I got slowly on a corner, kept myself holding by the arms on the sides of the pool so I could keep on the water and not drown (didn't knew how to swim on that age) even as my feet couldn't reach the ground.
Well, my brother kept teasing me because all I could do was stay there holding myself, so I tried to hold the least I could and try to reach the pool floor with my feet, when I was just with my nose not on the water with my head up I could actually feel a little bit the floor to wall intersection, that was like this /, at an angle, so I could touch my feet on it barely even as on the actual floor I couldn't, so I started screaming to him, look, I can touch the floor! He just didn't gave a fuck and went away, he already knew to swim, so he went to swim, so I got enough of it and started trying to get up again, and what happened? That intersection was at an angle as I just said, so when I tried to take some impulse with the tip of the feet to go up again so I could place my elbows on the size of the pool again and have enough strength to get out, the angle made my feet slip, and I went underwater, without anything to hold anymore, without a deep breath as it was all of a sudden, without knowing how to swim, completely suprised, fear took control of me, I couldn't even open my eyes under the water, it was one of my very first experiences on a pool, I was on the dark, I opened my eyes and they started hurting but I was so shocked I couldn't close them, I tried moving on all ways, didn't had any notion of what to do, then I eventually shocked my feet with the floor, tried standing up, hadn't enough height to get my head underwater as I said, so I crouched to gather strength and jumped, I got my head underwater, could see I was still close to the side of the pool, and got almost nothing of air from it as I really didn't went over my head out of the water, then I tried and did it again, screamed to my brother, must have been diving because he didn't listen or answered, then I was really, really out of air and completely desesperated, then I did it once again with all I had and I could somehow hold my right hand on the pool side, pulled myself up to breath and hold with the other hand, started heavy breathing still very shocked, pulled myself backwards and got my elbows on the sides and got out of the pool, started crying, my brother then realized something happened with me (didn't cared at all, though, lol, he really hated me back in the day), then I went home, he got after me and told I couldn't tell it because he would get in trouble, or else he would beat me up (yup, hated me, lol).
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Year later, I was around 13, knew how to swim properly, and was there enjoying the pool, then the small sister of a friend got there, she was like 10, but heavy, and she wanted to play with me, my friends and I used to do this thing that we would go underwater, let the other keep floating over us with his feet on our back, then we crouched and jumped from the pool floor to make the friend go up off the water into the air and fall on it (you get really stronger underwater).
She wanted to play this game, I went and did it a few times with her, she got really happy, then eventually we stopped, the thing is, I was still there, close to the pool side walking a bit when she, from outside the pool, just jumped inside the pool right over my head, from behind, I just felt a heavy hit and fell underwater, in a flash, but what's more, she fell over me, and she just wasn't getting away, I was underwater, without breath, with she sitting over the back of my head, I started trying to take her out but I was in a really confused state and bad position, I just couldn't take her out, I started panicking, almost ran out of air, but eventually I got my senses a bit better and I really forced her off me and jumped off the water to take a desesperate deep breath, and get really mad with her, lol, I started shouting and she started crying, other people started looking at me, I turned into the villain, just left the pool, and she actually never came again to play with me... or try to kill me XP