Finally I'm getting closer to try out ReefMaster.
Since I'll be testing it on the Adriatic sea in Italy I'm looking forward the best way to draw the shoreline.

First of all let me tell that both C-Maps and Opneseamaps contours are not really accurate as I need, since there are many recesses and tips that are drawn as strict lines.

So, following the Vimeo tutorial I see that it's used the poligon tool in Google Earth to draw the shoreline.
But since I need an open shoreline, would the path tool work as well?

Let me make some guesses:

considereing what Matt told on another thread, Reefmaster would eventually close any open path, so I think that using the path tool may work ok.

Just making the two ends of the path go far away from the shore should do.
Hope I find the time to try RM.

That's exactly right. ReefMaster expects all shorelines to be closed polygons, and will close them itself if they are not. So create the sea shoreline as you have done, and then join them using a few points just as you have in your image. When creating the map, define a mapped area (red box, below)
that lies a good distance within this defined "shoreline" so that the outer limits of the shore do not provide zero depth values to the finished map (i.e. in the image below, make sure the difference between the red and green lines is greater than "max interpolation"):