Lagoon

Etymology

From Italian "laguna," from Latin "lacuna" ("pool, gap, hollow"), from "lacus" ("lake"). The same "lacuna" gives us the word for a gap or missing piece.

noun · /luh-GOON/
A shallow body of water separated from the open sea by a reef, sandbar, or coral ring.
What it names
The calm, sheltered water behind a barrier — the tranquil pool of the tropics.
Example
The reef sheltered a still, clear lagoon of pale blue.
Related idea
A lagoon is calm water held back by a barrier; a fjord is deep sea water reaching far inland between cliffs.