How can I duplicate a bay with its coastline and close the Polygon with a straight line in QGIS?

Question

I am novice in QGIS and I am trying to find a way to copy/cut/clip a polygon(coastline) to a new shape and new layer and close the polygon with a straight line. The new shape should contain at least the georeference and the calculated area of the lot.

In other words I have to enclose a bay in a new shape but close the bay mouth with a straight line as a new shape(restricted fishing area). This has to be repeated in many geographic areas. With CYAN color is the land and with WHITE is the sea. The RED is the enclosed area banned from fishing.

Answer 1

1. Create a new layer
2. Start editing, enable snapping and trace
3. Draw the bay following the shoreline

Answer 2

Using Menu Vector / Geoprocessing Tools, you have at least two (very similar) options:

Union tool (no auxiliary layer necessary):

1. Draw a polygon overlapping the area you want to copy on the same layer.

2. Run the Union tool

3. Select the polygons that do not represent the overlapping and delete them

Output of Union tool with red outline: delete no. 1 and 2:

Clip tool (auxiliary layer necessary)

1. Draw a polygon overlapping the area you want to copy on a new, auxiliary layer (poly2 on the screenshot).

2. Run the Clip tool with the two layers as input.

Red: output of the Clip tool