I was at a waterpark as a young teen and ended up trying the wave pool, but even being tall for my age I was shorter than a typical adult. When I first went out the waves weren't turned on/going yet, but once the waves started everyone moved forward and the crowded pool compressed even more, plus I got pushed even deeper into the deep end, and basically I sank down because I couldn't really get any space to swim and everyone else was standing. It was so packed with people that eventually (without my realizing at the time) it became impossible to move arms and legs enough to stay up, kinda like a crowd surge but in the water.
I must have been too shy to think of climbing onto the person next to me. My best guess is that I was "jumping" up off the bottom to get brief bits of air while hoping it wasn't in the middle of a wave. After doing this for a bit, could be just seconds, I started to panic (I really couldn't tell you how long, felt like forever). I heard a whistle and somehow this lifeguard was there through the crowd within seconds (they had been standing along the wall of the pool but I was more in the middle).
The people all around me shoulder to shoulder hadn't even noticed what was going on, I still feel amazed the lifeguard could pick me out from thousands of heads and get to me.
(I don't know if this can be considered "nice" cause it was their job, but it's something that has always stuck with me).