How can one draw polylines or curves in Google maps between two location but avoid drawing on land?

Question

As the question states, how can I draw a polyline or a curve between two points on a Google Map something like this:

but avoid drawing on any line between the two points.

I want to use this feature to draw dynamic cruise itineraries. Do not take into consideration very complicated routes, something simple.

For example assume I have this locations: Barcelona, Marseille, Nice, Genova, Napoli, Palermo and back to Barcelona.

If I draw straight lines it would look like this: http://prntscr.com/6eubrs Instead I would like to achieve something like this: http://prntscr.com/6eucm9

Or is there a way to use/access the ferry routes and draw onto them?

Answer 1

You could use OpenStreetMap and only import the ferry data with one of the existing routing engines