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Could be a stage version of "And Then There Were None"/"Twelve Little Indians". The female protagonist tricked a child into swimming out well over his head (figuratively and literally). A flashback to the scene is in the book (and the movies?) So, it would probably be in the play.
Does she remember where she saw it? That might make a difference.
Not sure where she saw it, but pretty sure it was a nice theater, so probably either Broadway or the tour of a show. When it cut back to all the characters in normal time/lighting, it sounded like they were waiting for the boy to surface, so the character was, most likely, not alone when they started drowning. There is a slight chance it was a musical, but about 90% sure it wasn't. Sorry for all the vagueness, but I can't really get in touch with the girl who was telling me this right now.
I've got a feeling it's Coram Boy which was shown on Broadway in 2007. That scene does sound familiar to when I saw it in London. It was a musical but it wasn't particularly very song heavy - considering part of the plot is based around one of the children being a singer. Based on the book by Jamila Gavin here
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What they did in that scene for the London version was have the blue lighting like you said and I definitely recall there being a sheet to represent the ocean. At least I believe so, it's been a very long time.
That's my suggestion. Can't think of anything else.
As for And Then There Were None. There hasn't been any productions of that outside of the UK... except in 1944.
Edited by Jicragg
FWIW, the production of And Then There Were None that my theatre did in New Jersey last year has no such scene. The character merely describes it in a dramatic monologue.
I got in touch with my friend and she said it was Coram Boy. Thank you for the suggestion!
Someone I know was talking about a stage play she saw, but couldn't remember the name of. The only scene she described to me was a young boy drowning. She said they flooded the stage with blue light while he pretended to drown in slow motion, before quickly going back to the other characters waiting, in normal lighting and speed. I think she said it was a fairly famous play, but I'm not sure. Any ideas would be greatly appreciated.