The most common and most likely answer to this crossword clue is the 10 letter word NAUTILUSES
I've seen this crossword clue in The Times
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Buddy explains!
The straight part
We know that "Sea dwellers with chambers/submarines" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sea dwellers with chambers/submarines = nautiluses.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does NAUTILUSES mean?
NAUTILUSES noun- A marine mollusc, of the family Nautilidae native to the Pacific Ocean and Indian Ocean, which has tentacles and a spiral shell with a series of air-filled chambers, of which Nautilus is the type genus.
- A kind of diving bell that sinks or rises by means of compressed air.