The major differences among sea, lake, river, channel and gulf are:
Sea-It is a massive water body that is saline in nature. Seas mostly occupy different parts of two or more countries or continents.
Lake-It is a comparatively much smaller water body that is either man-made or natural. It can be either fresh water or saline. Example: Dal lake, Mansarover lake (fresh water lake) Chilka lake (saline lake). It occupies a state.
River-It is a long water body or natural stream of water that empties itself in a sea, ocean or another river. It covers two or more states. Its source is mostly a massive glacier or a huge lake. It is either perennial or seasonal. Example: Source of river Ganga is Gangotri glacier and source of river Tsangpo is Mansarover lake.
Channel- It is a type of landform consisting of the path of narrow body of water, most commonly the confine of a river. Example: English channel.
Bay-Bay is a broad inlet of the sea where the land curves inwards or partially enclosed by land. A bay is smaller than a gulf. It is a water body partially enclosed by land but it has wide mouth, affording access to the sea. Example: Bay of Bengal, Hudson Bay.
Caution: Explosive. Handle with care.