Recently got a question about how to make a fish look "wet" underwater in a tattoo. Just thought I'd share the answer with all of you in case you are problem solving something similar.
Ok. Funny thing. If a fish is under water, it doesn't look "wet". Why is that? Because all of the light on the fish is diffused because it's passing through the water. If a "wet" fish is on a table however, it will have sharp, bright highlights. Sharp highlights denote gloss, soft ones denote a more matte surface.
My last piece of advice is look at photography of your subject, even if you aren't copying a photo. Reality has all the answers when it comes to light, shade.
Hope this helps!