UV doesn't throw very well. You could try using a UV drop-in...
http://www.manafont.com/product_info.php/taiwan-3w-380nm-uv-flashlight-dropin-module-42v-max-p-5697
with a smooth reflector...
...to see if that works for you. I've never tried on the water like you are attempting. I did add an aspheric optic to my 501b with the UV drop-in and it didn't work well. Either the UV is just too dim or the glass lens filtered out too much of the UV to make it effective. For my use, I use an OP reflector because I'm generally looking up close and want to cover a wider area: