almost totally

[ostap77]

Would it make sense if I said "Dry Tortugas National Park is almost totally underwater"?

 

[birdeen's call]

With a space: under_water.

 

[ostap77]

With a space: under_water.

My Webster says it can be also underwater without a space.

 

[Rover_KE]

My Webster says it can be also underwater without a space.

Your dictionary says underwater is an adjective - (an underwater grotto).

Here, under water is an adverbial phrase.

BC is right.

Rover

 

[ostap77]

Your dictionary says underwater is an adjective - (an underwater grotto).

Here, under water is an adverbial phrase.

BC is right.

Rover

"Meaning:
: located, used, done, or happening below the surface of water ▪ underwater caves/volcanoes ▪ underwater photography ▪ an underwater camera
—underwater adverb ▪ swimming underwater"

It says also underwater-adverb

"un‧der‧wa‧ter [only before noun]
below the surface of an area of water, or able to be used there:
an underwater camera
—underwater adverb:
He dived underwater and swam away."

http://www.ldoceonline.com/dictionary/underwater

 

[probus]

Merriam-Webster online appears to be your first source and it does include the phrase "underwater - adverb." But no example of adverbial usage is provided. The link just takes me back to the adjectival definition.

The citation for your example "He dived underwater and swam away" appears to be Longman Dictionary of Contemporary English online.

Okay so says Longman, but I have never heard underwater used as an adverb in my life, nor can I find an adverbial use in a quick search of the British National Corpus.

Don't use "underwater" as an adverb. Use "under water".

 

[ostap77]

Merriam-Webster online appears to be your first source and it does include the phrase "underwater - adverb." But no example of adverbial usage is provided. The link just takes me back to the adjectival definition.

The citation for your example "He dived underwater and swam away" appears to be Longman Dictionary of Contemporary English online.

Okay so says Longman, but I have never heard underwater used as an adverb in my life, nor can I find an adverbial use in a quick search of the British National Corpus.

Don't use "underwater" as an adverb. Use "under water".

underwater
adjective
adverb
/ˌʌn.dəˈwɔː.tər/
/-dɚˈwɑː.t ̬ɚ/

"under the surface of the water, especially under the surface of the sea
an underwater camera (= a camera that you can use under water)
Some species of turtle can remain underwater for 24 hours."

Definition of underwater adjective/adverb from Cambridge Dictionary Online: Free English Dictionary and Thesaurus

 

[probus]

underwater
adjective
adverb
/ˌʌn.dəˈwɔː.tər/
/-dɚˈwɑː.t ̬ɚ/

"under the surface of the water, especially under the surface of the sea
an underwater camera (= a camera that you can use under water)
Some species of turtle can remain underwater for 24 hours."

Definition of underwater adjective/adverb from Cambridge Dictionary Online: Free English Dictionary and Thesaurus

There is also a difference in pronunciation:

under water vs underwater

If I heard "under water for 24 hours" I'd know exactly what the speaker meant and consider it a fluent utterance. If saw in writing "underwater for 24 hours" I'd consider that a spelling mistake. If I heard "underwater for 24 hours" I'd assume the speaker was a person with a foreign accent.

Anyway, suit yourself. We've told you what we think.