The can either be used with noncount nouns or the article can be omitted entirely. For example:
She liked to sail over the water. Here, some specific body of water is being talked about.
She liked to sail over water. Here, no particular water is being talked about. It can refer to any water.
‘A’/’An’ can be used only with single count nouns.
I need a bottle of juice.
I need an eraser.
Use of ‘the’ in case of geography
There are some specific rules for using ‘the’ with geographical nouns.
Do not use ‘the’ before:
Ø names of most countries/territories: India, Brazil, Canada; however, the Netherlands, the Dominican Republic, the Philippines, the United States
Ø names of cities, towns, or states: Toronto, Delhi, Sao Paolo
Ø names of streets: Callowhill Drive, Park Avenue
Ø names of lakes and bays: Lake Michigan, Lake Ontario; except while referring to a group of lakes - the Great Lakes
Ø names of mountains: Mount Everest, Mount Fuji except with ranges of mountains like the Andes or the Rockies or unusual names like the Matterhorn
Ø names of continents: Asia, Europe
Ø names of islands (Easter Island, Maui, Key West) except with island chains like the Andaman Islands, the Canary Islands
Use ‘the’ before:
Ø names of rivers, oceans and seas: the Ganga, the India Ocean
Ø points on the globe: the Equator, the South Pole
Ø geographical areas: the South East, the Asia Pacific
Ø deserts, forests, gulfs, and peninsulas: the Kalahari, the Sunderbans