It bugs me to need to go around a narrow land mass when there's a just a narrow land between two sides. When there's 1 tile founding a city can be solution but most of the time it wouldn't be worth it. My proposition is a new improvement called canal, naval units can enter a land tile with canal. To prevent making huge canals all across the country i can think of two solutions. 1) canals can be built only next to coasts and lakes. 2) canals have really high maintenace costs.