Here's how the tiles currently work. There's a game exploit in M2TW, where you can kill off an agent by surrounding him with individual military units, and dropping one more unit right on top -- "poof" he's dead. Very useful for eliminating pesky Inquisitors and heretics if you've tried everything else, and just need that agent GONE from the map.

You have to put three units across the top, one to the left side, one to the right side, and three across the bottom (if I'm remembering this correctly). The agent is occupying the tile in the middle, so it's a 3x3 grid of 9 tiles where only one army or agent can be at the same time. The whole campaign map is overlaid (or underlaid?) with that grid, and it's how movement and blocking is determined.

So if they're getting rid of that grid, it might mean more fluid movement. Although I'm assuming there will still be some kind of "zone of control" so you can't just sneak past (or through) an enemy army blocking a strategic choke point on the map. Ideally that zone of control should be much smaller for fleets at sea than for land armies. Visibility is just line of sight on the ocean, so you don't block or control much area. On land, it's more determined by geography (i.e. one single mountain pass or bridge can "control" a huge area if it's the only way through geographic obstacles). There might also be spies and such that would let you know if an army was passing nearby, which isn't the case for ships at sea. I don't know if that's what they're actually doing with this game, but getting rid of the tile grid would allow that flexibility.