So I got to the part in the quest where you're supposed to dive in the flooded valley, and enter the laboratory through a portal underwater.

But the portal is not there, I know I'm looking at the right spot (as it is not really complicated to find it, and I looked it up the internet to make sure too). I tried travelling out of Touissant and back, and restarting \ reloading the game, but the bug persists.

Anyone else here encountered that bug?

On my first dive there I went into the opposite direction. You are sure that you did not do this, too?

I tried both dierctions, none work for me.

I'm having the same problem

not sure if I had to interact/interacted with it to open, but my portal opened right in front of me. And teleported right away.

Well, it doesn't open for me... And I can't interact with it too.

No need to interact for this portal, try using "your" witcher senses, bet it will show up.
Good luck!

Thanks, I tried that too. Alas, no portal still.

I am having the same bug, but I saw elsewhere where if you reload to a save before the mission it should work. The problem is that I just said fuck it and moved on. I did a lot of other side and main quests and it would be detrimental if I had to go that far back for these mutations.

Me too!