i tried out now my idea about a seafaring system,

it works that way:
The sea is devided in sea-sections.
Every sea sections is devided in 4 quarter, which all have the same connections and are connected to each other.
Sea sections are connected to bordering sea sections.
Most coast territories have harbours, which are connected to the accordant sea-section and their accordant land-territories.
No direct Land-Sea connections.

This combinated with a kill rate like 25/42 , represents 2 realistic facts :

It is hard to attack land from sea(you need to enter the harbour, the defender has time to build a defense)
It is hard to defend Seas (you need to defend 4 sea- quarters same time , so better retreat to your harbours)

Here is my example map: http://warlight.net/Map?ID=8361
say me what you think about it and what one could improve

That's clever... I'm going to have to try out your map, but just at first glance I really like the idea! I like the quartering system, and I especially like that the seas aren't part of any bonus so they're only useful for transportation.

I'm also working on a similar seafaring. However I dont really agree on your basics. Defending a sea from other fleets completely would be hard, but moving a ship to any location will typically be easy. At least in historic maps where you dont have spy satelites and such items. Also ofc it would be hard to attack a city from sea, but it would not be much harder than attacking it from land. A sea attack would probably be more of a surprice than a land attack. Again assuming you dont have spy satelites ...