How many letters does the top answer for “large watertight chamber used for construction under water” have?

The most common answer is caisson (7 letters).

Crossword clue: large watertight chamber used for construction under water

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

caisson 7 letters wn-def

large watertight chamber used for construction under water

cofferdam 9 letters wn-def

large watertight chamber used for construction under water