I can't keep a secret very well or for very long.
To get the wet effect for my entry into the GGW photo contest, I used vegetable oil. I dipped the blooms and leaves in a shallow dish of oil, carefully placed them on the scanner, and then brushed a bit of oil around for the ripples. Vegetable oil is quite clear especially in small amounts, and it's viscous which means it's easier to control. I was careful to not get it near the edges, and when I was done, I carefully sponged it up with paper towels and cleaned the glass with a paper towel damp with soap and water. The final clean was just a bit of rubbing alcohol to remove any bits of dust and soap residue. There weren't any ill effects to the scanner, and it's working just fine. If you try this, you will have to be careful, but it's definitely better than trying to use water.
If you try it out, please let me know! I'd love to see your results!