Im just wondering how important the respective endurance perk is - e.g. if there are many underwater areas worth exploring? Please no spoilers - just some idea

How do you expect to get an answer to this without spoilers?

Without giving anything away, it could end up a lot more useful than you may think. The second rank even has a very interesting strategic side-benefit for anyone who likes to play as a stealthy character...

I have no inside information, so I CAN'T spoil.... but I have to say, if there ~are~ large parts of the game underwater, there almost certainly has to be a rebreather you can create / find. There's no way they'd gate a LOT of content behind one perk. I had not planned to take END higher than 1, and Aquaboy was the only perk that tempted me. The second rank (from what has been revealed publicly, I think) makes you undetectable while underwater. This could be handy, true --- in previous games, when underwater you cannot "sneak", so you would splash out of the water onto dry land and THEN sneak, by which time everyone nearby could detect you. The times that this will be a factor, though, aren't worth me sinking four stat points and a two perk points into Aquaboy, IMO.