No I don't watermark - I find it intrusive, and so would my target audience.
Instead, I post images no more than about 800 px along the long edge - which is far too small to allow a sharp print larger than 4 inches! If it's being used on the web without my permission, good luck to them - when I encounter anyone doing that, rather than getting on my high horse, I simply ask them for an acknowledgement and a link to my website.
What I don't get is photographers who carelessly allow large images into the wild. If you Google many well-known photographers, you can often find huge files. For example, the Magnum photographer Lise Sarfati,
has several, including my favourite image of hers, as a massive 14 MP file - it's the very first image! Granted it's probably not the photographers directly at fault but their agents or a gallery, but all it'd take them is just a few seconds to search and check...
Anyone want any Cartier-Bresson's to hang on their wall? Though I'm sure you wouldn't print them, being respectful of others' work...