Being blockaded has never been a problem for me. I always experience a quick surge of expansion in the first 10 turns or so with every faction and as a result I always have too many ports for a blockade to hurt my income in any significant way.

Keeping a defensive fleet would cost a lot more than randomly getting blockaded every now and then. (At least so far I've never seen an AI sustain a blockade for longer than 5 turns.)

As a result I always cut my costs by disbanding all my fleets on turn 1 unless I need to transport troops/dudes in the early turns. (For example, as Venice, I always give Iraklion to the Pope on turn 2, since the Bysantium seems to always use a lot of resources to attacking it and defending it is definetly worth the cost. So I let the Pope get angry at the Byzantines.)

It is always better to make savings on your resources than apply pressure on anothers resources. Since resources saved you can use against anyone when you so choose, but resources expended need to be directed at only one enemy. As such directing, those resources as efficently as possible for maximum profit at maximum operational speed is a critical way of saving florins.

Due to units' upkeep costs, the later you have to commit florins to a military effort the more you save and the more resources you will have when you finally decide to commit. Disbanding a unit (one you don't have an immideate need for) and recruiting a similar unit again later is cheaper than keeping the original unit around for 3-5 turns depending on the unit's costs. It's all a big game.