There are UVA, UVB, and UVC. UVA is commonly used for light effect and rarely cause any long term health problem. UVC is what sterlizes but it can also burn your eyes like arc welding tool. I have a 12" UVC bulb used for EPROM erasing, the bulb looks clear and if you have safety glasses it produce something like light blue-purple glow.
For water cooling, using UV light is not practical as it'd be too hard to build a rig that is also safe and shields completely from any passing eyes. Just stick with chemical that prevents corrision and kills orgisim. True some can escape chemical poison but if you do check the reservior or tube and flush/replace regularly you shouldn't have anything to worry.