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 large arm of a body of water partly surrounded by land is commonly referred to as a "bay." Bays are typically characterized by their calm waters and can serve as important habitats for marine life, as well as providing sheltered areas for boats and recreational activities. They vary in size and depth, and their shapes can be influenced by geological and climatic factors.
a sea
A large body of water that is partly enclosed by land with a wide mouth leading to a larger body of water is known as a bay. Large bays may also be called a gulf, sea, sound, or bight.
a large body of water surrounded by land.
A lake.
Lake
A lake is a large body of water surrounded by land.