Some kind of boiled linseed oil. It’ll get marks but spot sanding and more oil will take care of them if they are noticeable enough. If the water just sits on it for a short time it won’t really care up to like a few days, longer sitting water e.g. around the sink will stain slightly and get rough.

Cologne has more than just Domplatte which is very full, although it is hard to find any that are not at least somewhat full just because the city already has 1M and there’s maybe up to 2/10 of that in visitors around that time as well. I’m of the opinion that none of these are worth it at night because it’s too crowded for me. On a weekday earlier than about 4pm they can be quite enjoyable.

• Domplatte : right next to the Dom and very full almost always, skip-able unless you need Dom specific merch
• Heumarkt - Alter Markt : Center of the old city often with quite cute stall design and ice skating on the Heumarkt this is where I’d go first because after the Dom it’s the first to get crowded. The stalls are good and the location is nice so I’d say take a peek.
• Schokoladenmuseum: Quite nice and on the water has the ferris wheel typically, not that coordinated but still nice. And not that central so a bit more bearable in terms of crowds.
• Neumarkt and Rudolfplatz: I end up hushing through these mostly Can be nice but they are both smaller and more geared towards food

There is at least another 5 in the city I’ve never been to