On the site of NFN (dutch federation of naturists) you can find a map. Just type in Amsterdam to see which locations are close.
http://www.nfn.nl/locaties/

In your case I'd suggest 'de molen', as this is a naturist club. This way your first timers might feel more at ease. I don't know wether you''l be admitted when not being a member of the club.

That's the huge advantage of the swimming pool, as entrance is public. This means: no membership card needed. Everyone who likes to swim in the buff can go there. This might be an eyeopener. And, they can quickly jump into the water and slowly get used to being naked amongst other naked people who don't bother about other people seeing them naked.

Thanks Bart. Will check out NFN and look into De molen. Possible problem here though may be that if it is labelled naturist my wife will sure twig and that will be a no go!  What's bruges like for naturism?  We're passing through there!

There's only one small naturist beach in Belgium, at Bredene, which is at about 30 min from Bruges. It's better to go to a naturist beach in Groede, Holland. It's a very nice beach, with at each end a beach house. So no problems with having to eat or go to the loo. Or you could go find yourselves a place near the boundaries of the naturist section. I've seen it before that after some time people are getting less reluctant to take off their kit. Even on the naturist section people wouldn't mind her being in a bikini and would immediately understand your situation.
In case you're interested, I could give you the GPS coördinates.

When are you coming to Bruges or passing by?