Not everyone knows how to swim. Also, you must not know how to swim either since you don't seem to have ever heard about undercurrents. Ktbn, da.
It's not about knowing to swim or not, but about the basic survival instinct. If you were thrown into a body water, wouldn't you try to get to surface even if you didn't know how to swim? At such a time you wouldn't think about if you can't or can swim or if you got caught by undercurrent, you would just do your dank best to get some air. Not like the MC here that just did shit and drown like a rock instead...
To me it looks like they simply skipped the struggling part. It only shows a frame when she just fell into water, a close-up of her desperate face, and in the next one she is already dying. There are pretty strong censoring rules in China, maybe it's that, maybe the author just doesn't wanna show the drowning or maybe they just decided that that would be a waste of space.
Btw, this is a pool. There are no strong undercurrents. To die in a pool where those floating barriers are within grabbing distance no matter where you at, is a bit of a strech, but I always knew how to swim, so I dunno.
God, I really must be a masochist if I'm still reading these Chinese shoujo #－.－) seriously, that was one of the most pathetic deaths I saw so far... "Oh no, somehow I ended up in a water, no use to try to swim now, I just float down to my death..."
Also, I usually don't comment on the translation, but it's kinda hard to understand the meaning...