Oh yeah, its ok, i recheck the pedia and sheep close the question
About rivers in two tiles island, i dont like the aesthetics, will be functional, but in game, the rivers are to big, and a two tile island will be desproportional with a river inside.
i think a three tile island (with a hill or mountain) will be more realistic.
If the memory dont trick me, in c2c, improvements in island, with a road will be conected with he rest of empire.
A better solution its ask to modders a new improvement what can be built together with normal improvements in islands or in water near island. A improvement like "foreing port" or something like this.