Another idea: Naval forts
Basically they can be built in coastal provinces and are very expensive. They do 2 things: 1) they provide a significant Fire bonus to allied ships fighting in the sea zone connected to the province's port. 2) They're like land forts, in that they can be "sieged" by ships, preventing blockade until the siege is broken. While a naval fort is intact, enemy transports cannot land troops in that province.
These would be particularly useful for island provinces, countries with small coastlines, and in important straits and channels.
Basically they can be built in coastal provinces and are very expensive. They do 2 things: 1) they provide a significant Fire bonus to allied ships fighting in the sea zone connected to the province's port. 2) They're like land forts, in that they can be "sieged" by ships, preventing blockade until the siege is broken. While a naval fort is intact, enemy transports cannot land troops in that province.
These would be particularly useful for island provinces, countries with small coastlines, and in important straits and channels.