An old black and white fim, not the Marx Brothers as I have all their films. It is a wedding or dance and the dance floor is a decked-over swimming pool.

The dancers are dancing and then someone throws the lever and the decking retracts. Most notice but not all, and in the water they go.

This has been bugging me for years, hopefully the internet can help.

Thanks in advance

Didn't that scene take place in It's A Wonderful Life?

"it's a wonderful life" swimming pool - Google Search

I remember seeing a bunch of Marx Bros movies back in the 70's but couldn't tell you what ones were about what.
Here is a list of all their movies

Movies - The Marx Brothers

Spot on, well done, thanks a lot, P.