LAKES are small, enclosed bodies of fresh water — too small to be the open sea. A lake tile yields food and trade like the ocean, and a FISH resource yields still more.
Their fresh water lets adjacent land be IRRIGATED, even far inland from any river.
A city beside only a lake is NOT coastal: it cannot build a HARBOR or the sea wonders — LIGHTHOUSE, COLOSSUS, MAGELLAN'S EXPEDITION, and DARWIN'S VOYAGE.
Lakes are calm waters: unlike the open ocean, they never spawn HURRICANES.