Water for a cpap

Yes, distilled water can be found in grocery stores near the laundry products (sometimes has a picture of a steam iron on the container).

You can also just use bottled water that has a low mineral content.
Normally, supermarkets carry a house brand that is just fine and won't harm your machine.

Thanks for the answers. I feel much better now.

I don't know what a cpap is, but do its instructions provide any guidance as to the appropriate water quality needed for proper functioning? It shouldn't be that hard to compare those requirements to prospective sources of water. In Europe, the producers of bottled water typically list a general and superficial list of chemical constituents contained in the product on its label. If a cpap involves heat, then the most common chemical properties relevant to boilers are the chloride content, the hardness, and the total dissolved solids. Those are commonly posted on labels (though hardness may need to be estimated from calcium and magnesium concentrations -- do a little research beforehand to understand the relatively simple calculation).

Here's a thought: I bet you could get in touch with your cruise provider and ask them to provide a bottle of the kind of water you need. It's hardly a frivolous request since it's for medical reasons. There's also a chance you could get that water through security if you had a note from your doctor explaining that it's a requirement for your medical device. People are allowed to carry larger bottles of contact solution, breast milk, and other medically-necessary liquids.

The cruise company should have a "disability desk" for you to make the request if you, or your travel agent, call in advance. They usually supply guests with a bottle of distilled water.

As others have said, distilled water is sold in grocery stores near the ironing supplies, a liter for about 50 cents. Otherwise, it's in liter glass bottles (about 4 euros) in pharmacies. The word in French is démineralisée (de-mineralized), so it should be similar to either the French or English in Amsterdam--and of course little language difference in Scotland, we presume.

Normally, you can buy demineralized water in plastic liter bottles. Depending on the length of your trip, you might just need one bottle.
A CPAP machine is designed to offset sleep apnea. Many people travel with them now.
There is not enough heat generated to make mineral deposits. If you wash the tank correctly with dish soap and warm water, it will take care of itself.
My brother just uses Parisian tap water and has had no problems.