The reality is that no matter how low the resolution you post, it's probably going to be suitable for someone's on-line useage. A watermark that runs across the image is about all you can do, short of simply not posting. Personally, I love sharing my images, and have no issues with non-commercial sharing as long as my watermark is left intact.
Having said that, I have had a number of images "borrowed" and used in dive shop promo flyers etc... sales, trips etc. although in all cases, the watermark is intact. In one shop's case, the accompanying text suggests that somehow that particular shop was involved in capturing that image... and they were not in any way. I mentioned it to the owner (who I know well and is something of a friend) and he sort of laughed it off with some comment about all the exposure I was getting. Awesome. 'Cause flattery is everything.... I take an altruistic approach in that if my borrowed images encourage someone to take up diving or something, than that's ok.