What you're describing could be called an inlet, a fjord, a marsh or a sound, plus other variants. The geographical description depends on the shape of the land that surrounds the body of water.
The water can be either salt water or fresh water, depending on the body of water's proximity to the sea or melting snow or springs.
Please see the link, below, to learn more.
Peninsula
A mere, a lake or an inland sea.
A bay is a body of water that is partly enclosed by land (and is usually smaller than a gulf).
a bay
a bay
a sea
That would be called a bay.
A lake is a large body of water surrounded by land.
a large body of water surrounded by land.
Lake
A lake.
Lake.