I missed the talk from Frogboy about no tactical water combat. Was aerial combat mentioned as well?
In a couple of the previews at other sites he mentioned that naval combat would be simplified. I believe somewhere on these forums he also mentioned that he's opposed to flying units, but I may be wrong. I'm sure there will be units with wings (dragons, for a start) but I don't think they'll be able to remain out of reach of ground units during combat. To be honest, that alone was responsible for some of the cheesy tactics in MoM (invisible flying warships anyone?), so perhaps it's better that way anyway.
I don't know if they'll be able to fly over ocean squares on the strategic map, but I would think so. It would seem strange to be loading dragons onto transport ships. I've got no problem with individual special units like dragons and so forth being able to traverse ocean squares easily, I'd just prefer not to see large armies able to do so. Even in the Silmarillion, I don't recall any such thing happening.
However, remember that we're being promised absurdly powerful Populous-style spells in the late game. I wouldn't be surprised to see spells that can raise the land to create a land bridge for your armies to traverse, or a spell to temporarily part the sea and hold back the water for as long as you're willing to expend power on it while your army passes though. I'd have no problem with that, and it would be quite cool. I'd just prefer not to see regular army units that can freely traverse ocean squares throughout the entire course of the game.