Aye. I'm overall very happy with 5e design, but this does feel like a pretty likely situation that wasn't addressed. Even a sentence chucked on to the holding breath rule would have been enough.
A few things:
First, if you drop to 0, you’re making death saves, which circumvents the suffocation rules except for the addition that you can’t be stabilized or healed until you are pulled up out of the water.
Second, if you’re affected by sleep , you begin choking and have from a minimum of 1 to CON modifier rounds for someone to wake you up, or you’ll drop to 0 (see above).
Yeah, I definitely don't want to kill anyone - but I want that threat to be on the table.
1 to 5 rounds isn’t immediate.Is dying, especially with no save, fun? No. Is it realistic that you die immediately if you are unconscious in water? Again, no. Even unconscious there's an automatic response that stops you from breathing in water. People can even "dry drown" with no water in their lungs.
It's one thing to challenge players, it's another to roll a bunch of d6s and say "Gee, you fall unconscious in water, guess you're dead."
So what if the PCs get death saves? Even if they make it they're still unconscious in the water.1 to 5 rounds isn’t immediate.
Also, death saves ARE saving throws!