I've had this scene stuck in my head for many years now, but I can't figure out what movie or tv show it's from. In the scene, a woman goes underwater in an indoor swimming pool in a house (which may be haunted). She looks up from underwater and sees many people standing at the edge of the pool, looking down at her. I am pretty certain the people standing at the edge of the pool were dressed like soldiers or from some time period before hers. However, she goes back above water and there is nobody there. The whole scene is tinged green I believe, kind of like in a Matrix way.
You may be thinking of The Legacy (1978)
It has a scene where a woman swims in an indoor pool, gets trapped under the water and drowns. It's like there's glass over the surface and she can't break through. She looks up and sees one person looking down at her, and he isn't in period costume, but it is tinged green.