some more points on blockades, mateys:
dont forget that one ship can blocade multiple cities within its effective blockade range. (is it a 6 tile-radius?)
while a ship is blockading it will not heal, so send yer pirates in pairs if ye can.
use one privateer to maintain the blockade, and the other to experience-farm on all the helpless caravels the AI sails around ye! soon, you'll be farming GGs and have monster ships to unlock west point and to upgrade to destroyers in a few years
you only gain gold per turn per blockaded city using privateers, and only then during peacetime. during wartime, the trade-routes and water tiles are still denied, but no gold per turn.
blockade all yer enemies ports, and he won't have access to overseas imports, yaar!
and last, don't be blockading a water-dependent city ye are about to plunder, 'cause once the port be closed, it shrinks in size faster than ye can imagine!