I think it's going to be very very very different from Oblivion.
It's well-known Oblivion's environments, dungeons (well basically everything) lacked of a unique and truly detailed touch, and there were never really any hidden things. Fin Fleam was pretty much the only secret in the water in entire Oblivion. Rest was just sand and rocks, and a slaughterfish every mile or so.
I mean I hope it's different, but I'm just not going to get my hopes up. Just by them having Fin Gleam in Oblivion they could say the exact same thing, "there's hidden stuff to find underwater".
Also, caves could be hidden by having them in places which are very hard to notice or reach. Ships could be sunk or behind some island far away, which you can't see from shore. There are tons and tons of possibilites.
But this is completely destroyed by the fact that a map marker would pop up. (if they are in) I remember in FO3 there was a cave that was buried into rocks, and I would've never noticed it but a map marker popped up and told me exactly where it was.
If you're referring to compass icons, I personally think they're gone or at least optional now.
Not just the compass, but on the map. It's too early to tell now if they're gone, but with the system they're going with (an even more GPS-like map) it doesn't seem like they'll be going away anytime soon.
Yea, Todd also said we'll see glacier dungeons, which could very well be hidden.
I can't wait to find the dungeon in a glacier, sounds so awesome!