Stance matters for attempting to engage an enemy force. Unlike land, there is no "control" in the water/rivers, therefore no automatic switching to attack mode. So 2 fleets will sit there never doing much of anything to each other unless 1 is on attack. As En91 says there is a cohesion penalty for being in attack mode however.
As for bombardment, a word of caution, you can ruin a perfectly good fleet bombarding a fort (especially in 1.15). Also, if you have land units riding with a fleet that is bombarded by land forces, you can suffer hits to your land forces (I know, it happened what a perfectly shamefully waste of an artillery unit :bonk

. Attempting to bombard greatly increases your chances of being hit by the land forces.
Riverine gunboats aren't useful for much, you do better with ironclads/monitors. The gunboats cannons are too short range to bombard, and they are too easily sunk during combat.
Remember - The beatings will continue until morale improves.
[SIGPIC][/SIGPIC]