I don't think making the AI more likely to transport troops by sea would result in spamming ridiculous invasions everywhere so long as factions maintain fleets patrolling their coasts. This is a reason why the new "zone of control" feature is so good. Unlike M2TW, when Portugal would show up in Ireland with a handful of spearmen every so often regardless of whether you had a ship off Ireland's coast, you can now put fleets out to patrol your coast and the invading force will have to engage your fleets before landing their armies. That should ensure the naval invasions don't get too ridiculous, unless you don't guard your own coastline, which is your own fault ....