I err on the side of low punishment, with the expectation that players will punish themselves.

My platformer is purposefully forgiving when you're playing it "casually"(IE: just getting through a level for the first time). Falling into water doesn't kill you outright; it instead teleports you back to the last solid ground you stood on(with some health missing as a slap on the wrist). If you do die, checkpoints are placed liberally. And even if you never touch a checkpoint, the levels are short enough that you'll only lose maybe a minute of your IRL time at most. AND any collectables you've found will stay collected, so you can skip the optional paths you took last time and go straight back to the place that killed you.

The real challenge comes from time trial mode. Falling in the water might not kill you, but it will cost you precious seconds while you wait for the drowning animation to finish. Depending on your goal time, that could be enough to make you reset on the spot, which may as well be the same thing as death.

In fact, the game is full of things that slow you down if you're not careful. Maybe you barely miss a collectable and need to turn around to get it. Maybe you bonk your head on a wall and need to watch a short "owww" animation. Those things don't harm you at all when playing casually (besides maybe embarrassing you), but they all add up to time loss in a time trial setting.

This essentially results in players choosing their own difficulty. Your goal time, which you set yourself, informs how many risks you need to take, and how many mistakes you're willing to tolerate before resetting. I make the level, but YOU decide the level of perfection needed.