*Pssst* Civil War II ...
That being said
brown water blockading coastal harbors is more effective
, but also take much more effort on the side of the player, mostly because you will need to relieve fleets on blockade picket when their supplies run low and/or their cohesion (low cohesion ships are vulnerable to attacks.
I've never tried to figure out what the cost difference might be, comparing how many ships you would need for each method, but to effectively brown water blocked, you will also need safe harbors close to each blockaded harbor along the coast for the pickets to retire to rest and refit (resupply and recover cohesion). These harbors will require garrisons, some fairly sizeable, to protect them from recapture.
IIRC the highest bluewater blockade value you can achieve without capturing any coastal cities is 50%. To achieve this you need at least 5 blockade fleets in each of the two blockade boxes (always evenly distributed, because only the lowest blockade value in either one of the boxes is evaluated.
Oh, the bluewater blockade does not use the 'Power' of the fleets, but their blockade value. Each ship/boat has a blockade value, which you can discover by looking at it in the element detail window. IIRC Brigs have a blockade value of about 2, while blockade ships have 10.
Brownwater blockading only counts combat ship elements, so and ironclad is equally as valuable as a gunboat in brownwater blockading, except when it comes to defending against the CS attacking a blockade.