Under water VS underwater

[Freeguy]

I have a question about the difference between "underwater" and "under water". Are they interchangeable?
For example in this sentence : This man is swimming ..... . [can we use both underwater and under water?]

thanks

 

[Grumpy]

Being very old and set in my ways, I tend to use "underwater" as an adjective [as in an "underwater camera"], and "under water" as an adverbial phrase [as in "this man is swimming under water"]. However, more and more people seem to use "underwater" in both cases these days, so you can decide which suits you better.

 

[riquecohen]

I agree with Grumpy, but I feel that they should not be used interchangeably.

 

[emsr2d2]

I would say "He is swimming underwater", but also "He is swimming under the water".