Imaculata
Hero
It's when they don't self-limit like that - going back to the trap example again but let's change it up a bit (water is too easily beaten in 5e): make it a trap where the away scout falls 30' into a pit of acid; the players know this but the characters do not and can not. Party happen to have a scroll of Protection From Acid on board - one-shot, gives everyone within 10' acid immunity for half an hour. While searching for the scout they reach a pristine hallway and don't (or can't) for whatever reason notice the trap. Do eyebrows go up if only now does the party use the scroll, even though there may still be half the adventure left or more?
So let me get this straight... the party splits up, and one character falls into a trap that the other characters have not noticed yet. Should they be allowed to act upon this knowledge as players?
Well to me, the answer to this would be to simply not put them into this situation in the first place. I would give the other players a clear clue that warns their characters of the trap, so they don't make the same mistake, and don't have to fake ignorance. Because I like it when there is not a huge mismatch between player and character knowledge. Avoid the situation entirely. Someone has already fallen into the trap, so reveal it to the other players clear as day.
As for the underwater adventure question: while it's quite true that the 5e version of Water Breathing allows for ongoing underwater adventures* (which is just fine; kinda cool in fact) I still maintain it's a bit broken the rest of the time as by in effect being permanent it outright neutralizes what might otherwise be a legitimate obstacle and sometimes hazard. It also kinda ruins maritime ship-board campaigns or adventures as perhaps the biggest ongoing hazard there is somehow going overboard. Can't swim? Who cares, you can't drown either.
This is my biggest issue with it as well. Of course since I am currently running a ship-board campaign, this sort of thing is a lot on my mind. One of the biggest threats in my campaign, is water. And I want to keep it that way, which is why I'm also keeping any magical items out of the campaign that would completely negate the threat of water.
A Druid shapeshifting into a marine animal is fine, because then his class ability is offering him a means to deal with the obstacle. That is the key element in my opinion: I want my players to interact with the water. I want it to inform some of their decisions. If they feel the need to reserve a spell slot, bring special items, buy a boat, or use some sort of special ability to deal with it, that is perfectly fine. But I don't want a spell that permanently makes water irrelevant for the entire party, and for anyone they bring along.
I want my players to wander into a dungeon, encounter a flooded tunnel, and ask their mage: "Did you remember to bring water breathing?", instead of the mage saying: "Pffft! My waterbreathing spell is still active on the whole party for the entire day, and when it ends, I simply cast it again. Just assume that it is always active."