the AI naval behaviour can be a bit weird - their ships just seem to wander aimlessly around until one day they decide to randomly blockade one of my ports and start a war for no reason (as is well documented)

I tend to keep one or two decent stacks of ships around for transporting troops and lifting blockades (although I prefer to do that with diplomacy where possible) - I have to say I've never considered it necessary to dominate the seas in order to win the game, but it is almost too easy to economically cripple the AI with blockades if you feel like it...