You won’t be able to sink, but you don’t need to be submerged to drown.
Odd. You really don’t need to know how to swim if you’re in saltwater with long fins. I’ve been snorkeling and scuba diving, yet sink to the bottom of a swimming pool.
I probably count as a non-swimmer: I can’t tread water. I can maybe do 50 metres front stroke before I’m thoroughly exhausted. I could probably backstroke indefinitely but I don’t know how viable that would be on rough water.
I’m not scared of the water as such, but one anecdote:
One time I was in a public pool, and up to then every pool I’d been in had been either all one level, or had a gradual decline to the deep end.
But this one simply had a sudden drop, so I was actually walking with my head just out of the water when suddenly – where’s the ground gone!?
And, I just freaked out. Madly thrashing and kicking for I reckon 30 seconds (it felt like minutes but objectively I can’t see how it could have been longer than that).
By the time it occurred to me to simply let myself sink a little, so I could turn my body horizontal, I’d started to swallow water and really felt like it was a “near-miss”.
I’m sure this would have played out differently for someone that could tread water, but the point is you can panic even if you have some swimming ability.