can i move troops across water in the campaign?

[Alexios]

The manual describes this, but it's not entirely clear...

To move troops from one province to another by sea you must -

a) have a port in the province you wish to leave from
b) have an unbroken line of your ships (ie. at least one in every sea area) between your province and the province you wish to attack.
c) no ships belonging to the faction you are attacking in any of these sea areas.

I believe if you are currently NOT at war with the faction at the time you move your army across the sea, their ships will not block your passage; only those factions you are at war with at the time you make the move, can block you.

Of course, once you do attack the faction (i.e., make war with them), you will not be able to move an additional stack to the target province, nor will you be able to freely move additional units after the battle is over. So you will need to deal with their ships eventually. Sometimes this is an important thing to consider prior to attacking, because your troops will invariably need reinforcements once the battle is over (the AI loves to counter-attack, esp. when you're besieging one of their garrisons!)

By the way, welcome to the Org, Better_Half!