One of the things I appreciate about Dungeons and Dragons Fifth Edition over any previous edition of D&D is the simplicity of their rules regarding underwater adventuring. Trying to fight something underwater? It’s either impossible or you’re bad at it. Unless you’re used to being underwater due to exposure or training, have magical aid, or are a type of person who just lives under water. In previous versions, there were a lot of rules about the types of actions that you can take under water, how shooting things works, how to attack things when the attacker or the attackee is out of the water and the other one is in it, and then all the fiddly little compounding numbers that come out of those declarations and determinations.
I remember one session I ran in 3.5e where the players found their way to a dungeon entrance, fought some creatures that fled into the dungeon, and then decided to come back in the morning because they were tired and wanted to hit the dungeon fresh. I, of course, altered the dungeon on the fly because suddenly the creatures inside it had time to prepare for a band of adventurers whose abilities they had a basic grasp of. The first, and most-damning, protection measure they put into place was filling the underwater entrance with a summoned creature meant to protect said underwater passage.
In this old version of D&D, the rules for drowning were basically “You can hold you breath a bit and then you’re unconscious, dying, and then dead.” There’s some rolls involved there that make it seem on paper like you have a decent chance of pulling through, but that’s typically not the case in practice. Since I didn’t want to pull my punches (one of the characters had deliberately sacrificed his life a few sessions prior to let the others escape an enemy, so I wasn’t about to lower the stakes) and I didn’t want my players caught off guard, I collected all the rules in one place for them, shared them around, and reminded them that they needed to get air before they ran out if they wanted to keep their characters safe. And to be aware of how their speed changed in the water. I gave them every chance to prepare themselves and even included mentions of what being underwater would mean for anything else they might care about. Like invisibility.
Anyway, one of them was certain he’d be able to get the roll he needed so he pushed further than he needed to which became an immediate issue because he made himself invisible and invisiblility literally doesn’t work underwater in that system (a basic rule states as much). He managed to escape the giant squid, but drowned trying to swim back towards the surface and air. Which meant that then the party halted for about a week while the fast-movement-centric archer ran their dead halfling friend back to someone who could bring him back to life. It was a whole debacle.
In 5e, this wouldn’t be a problem. The rules are simple: you get disadvantage as appropriate, there’s no specification about what magic works or doesn’t work underwater (though anything requiring verbal components probably fails if you cast it underwater), and your movement is reduced as you try to swim around whatever weird aquatic beastie you’ve decided needs to be pursued to its home. All around, a much simpler system. Still, the first time I brought my players to an underwater area, introduced water rules, and told them to be aware of how this stuff works, one of them still managed to drown themselves. They got over it, thanks to a bit of quick clerical magic mid-combat, but it was quite a moment to watch that player find a way to get themselves killed almost immediately despite the much simpler rules.
Just goes to show you that all a player needs to get themselves killed is one or two rules to ignore until it kills them despite my warnings. Good times.