Who knows the name of this movie where the killer is a scuba diver?

It is an old film, I’d say 70′s 80′s, probably foreign. It is a horror flick, the killer is a scuba diver. I don’t remember much, only that one of the murders takes place in a swimming pool, the victim is a lady on a black inflatable donut. Another scene I remember is when they drain a lake or quary, only to find the killer’s scuba equipment, not the actual killer himself.

sounds like Cabin By The Lake

Scuba Skill – How to Deploy an Inflatable Signaling device

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