you get blockaded in the sea areas where a faction you are at war with has ships . to see which areas are clear and which are not press 'v' while you are watching the map. unblockaded areas with your ships on them show as green , blickaded areas show as red.


....about the money. i cannot verify it but it might be a flunctuation because of an event since the decrease seems small. income reductions due to blockades are ussually large ,for example from 45.000 to 12.000 ( a good reason to have spare cash and a good naval power)