Thank you both very much for your extended feedbacks, very appreciated!
I had a discussion about this question on another forum and my close guess was that sailing at 2 knots with the scope deployed in rough conditions should be a safe limit - but probably a useless action as for what you could actually see with all the splashes. My recommendation in the game UBOAT was: Go deck awash to increase your sight range a bit at the risk to be spotted.
My best greetings to you
XS