Seeing as the land combat as not changed very much from EU3, i don't really get my hopes up for naval combat, but i wonder what improvements are coming, because in my opinion naval warfare in EU3 always was kind of silly, with huge doomstacks of warships sailing around, the pirates who were kind of a big deal in the era are just a minor annoyance really easy to deal with.
I think a great improvement would be taking some mechanics from HOI, such as Positioning and the way ships engage each other, ideally in a way that "simulate" the line-of-battle combat of the 1600-1800 era.
Having positionning matter for naval combat would avoid stupid doomstacks of 200 ships being too effective, as those ships wouldn't realistically be able to move en engage targets in a relatively small area, and worse if we add friendly fire penalties,
Basically this would restrict stupid stacks, in a similar way that land battle have a front width that limit the number of effective soldier at a given time in a battle.
From the screens we have seen so far, it seems they kept the system identical to EU3, but i hope we can expect this will be improved later down the line...