How to build Arenas under water?

Ok, the question has a dumb answer: just go underwater with the admin vest and build the damn arena.
Ok, DONE.

But how to manage the players to NOT DIE underwater? Even if i close the arena with walls and ceiling, the players will die there.

And i saw several arenas underwater. How to make the players not die on them?

 

Thank you, Wulf, but this not what i meant... this plugin makes everybody capable of go underwater.
What i wanted was somekind of safer zone... but nevermin, i had a better idea and i will build underwater than copy-paste in the sky, as Reneb taught in his plugin videos.
Thank you for you answer anyway, Wulf and sorry i wasn't a bit more specific.

 

Nevermind... the sky arena solution is a much better one. And the view from the sky is awesome!