Characters have pool party.
But OH NO, THE READER CAN'T SWIM!!!

Most people know how to swim, and I'm a really good swimmer.
I swim competitively.
Only a small amount of your audience can't swim.
If you can't swim yourself, then feel free to write that. Because then you'll be able to make your character more relatable to people who can't.

But if you can... -sigh.-
First off there are stages to drowning.
It's not just oh, I fell in the water and sank to the bottom.

First off you struggle, trying to keep your head above the water.
Then you sink down slowly.
You then struggle again
And chances are you'll forget to hold your breath.
So eventually you'll stop struggling, and that's when you start to drown.
Depending on how quickly your saved, you may need cpr.

Sorry if I've offended anyone.