How many letters does the top answer for “beneath the surface of the sea” have?

The most common answer is submarine (9 letters).

Crossword clue: beneath the surface of the sea

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

submarine 9 letters wn-def

a submersible warship usually armed with torpedoes

undersea 8 letters wn-def

beneath the surface of the sea

beneath the surface of the earth 50% similar → underground