The model-turned-actress and the former 'Dawson's Creek' star - who she has been in a relationship with since 2006 - enjoy swimming nude together and always splash around in their outdoor pool when the summer months kick in.
Speaking to E! News at a celebration event of designer Prabal Gurung, Diane, 36, said: ''Well, I prefer to go (swimming) naked, but we have a pool, and actually our house is sort of in the middle of nowhere, so skinny dipping is still an option!''
An seemingly embarrassed Joshua swiftly turned his lover's revelation into a joke, saying: ''Well, not anymore!''
German beauty Diane - who has residences in both Paris and Vancouver with the 34-year-old actor - recently admitted she wasn't initially attracted to Joshua and actually couldn't wait for their first date to be over.
Speaking previously, she said: ''It was the worst evening we have ever spent together. Our first date was a real disaster. The worst thing was that he even tried to kiss me when he had taken me home. I said, 'Are you kidding me?' And then I rushed out of the car!''