Clearly you haven't kept up-to-date on Black Flag.
Saying that most of the game is at sea is blatantly [u]FALSE.[/u] There are over 50 locations in ACIV, ranging from Caribbean Islands to Sprawling cities. Verticality has returned after the lack of rooftops to parkour across in ACIII. Taking over ships and forts is completely optional.
We hardly know anything about the game yet to judge it. They have never released details of the story for any AC game, so I don't see why they would now.
And regarding the hidden blade complaint in your OP; just watch the videos that they have released showing the combat and stealth gameplay, and you will clearly see that the hidden blade is as prominent as ever.
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The islands are very Tiny and any casual player can cover a small sized island in under 15 minutes. I do see the hidden blade in action however Edward really likes using those pistols and swords of his for some reason...
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Caribbean cities are generally pretty small buddy. If you want a city the size of Rome in the days of old you'd be stuck a list consisting of many of the cities we've already been to. Embrace the beach life man. Where did I kill him? "Down by the beach mon"