As far as I remember, they're invisible until you're going next to the boat ("far" means 30 minutes ago, I just tried).
Rly? I thought they were sneaking ...
Since if you sneak close enought to attack, they appeared last time i tryed. O_o
But it was patch 3, so i take your word for it.
Maybe something changed.
Every exemples you gave rely on metagaming and being able to continue the game through another way doesn't prevent you to meet them.
Well ... yes, i used metagaming and i believe i even said so ...
But all that gives me is knowledge of where those encounters are, so i dont need to sneak through whole area exploring what surprises are waiting for me around the corner ...
Its totally possible to scout ahead in your first gameplay, as in your fifth ...
Its just easier in fifth, bcs you allready know where it will be just waste of time.
On the other hand, come on.
Completely empty village in the Underdark? If you approach it from norht, whole path paved with corpses?
Lets be honest with each other, if that dont raise at least one red flag, then nothing will.

All you wrote is "you're wrong, you can avoid combats". There are 0 arguments related to OP's point (overpowered encounters) except "metagaming".
On the contrary ...
OP is talking about hard combats ... i provide options to avoid hard combats.
Either by avoiding combat in general, or by preparation that allows combat ... but makes it a lot easier.
Sure, but remember our previous exemple ? You only talked about sneaking, pushing, remove OP advantage/disadvantage from highground to make combats "easier"... Not always... but often, it looks.
Well, after all its mechanic that is in game for you to use ...
If you dont like it and decide not to use it, its totally your decision ... and your consequences. :-/
OP sounded me like: Here are examples of hard combats.
I answered: Here are examples of how to make those specific combats easier.
Use them as you will, or dont.

Is that supposed to be easier if you scout or is that supposed to be very hard if you don't ?
To be, or not to be, that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles
And by opposing end them.Sorry, i didnt resist ... O:)
Cannot be unambiguously determined ... if you ask about my own opinion, it should depend of chosen dificiulty ... right now, we have only one, so as long as Larian take the data in the end and say something like: "many players died in this settings ... lets make it harder dificiulty" ... or on the contrary "many players survived without a loss of single character in this settings ... lets make it easier dificiulty".
I shall say it was good work.

Making things wrong and "learn to play" is fine and usual
Being driven to a specific way of playing is bad, especially in a tactical game and even more in a DnD game that's supposed to be about players creativity.
Agreed ...
The difference here is that i see creativity in scouting ahead, preparing your battlefield and expect the worse ...
And from some people (cant say everyone, but certainly some) it seem like DnD lovers see creativity in standing right in front of your oponent and simply take turns in bashing each other to head, until one of them fall on the ground dead.

HighGroundAdvantage=0;
I presume, it would look differently in actual game files, but ... you get the idea.

I mean, all you need is removing the effect itself ... AI would still try to get to high ground and behind your back ... but once you simply remove it giving advantage, it would have zero effect.

Its lazy solution and it have certainly many holes, like people would certainly start asking "why did that Goblin spend five turns to get on that hill, when there is no advantage" ... but i was allways lazy writer so ... that is probably why i will never do this for living.

Nothing changed by my side ...
As long as enough people take part in the survey, its usefulness is unquestionable.
And that's exactly what my argument was about ... if your opinion is supported by 500 people, then it is much weaker than if it is supported by 50,000 people.