I didn't really know what monster I was drawing when I started, but I'd decided on something underwater. My composition lines ended up looking like tentacles, so I ran with it. I noticed that the dark side of the water's gradient would be a good place to add bright contrast for the focal point. The needed details were obvious, so I used an opaque hard brush. This also worked out well for the specular lighting needed to make the tentacle look slimy. Finally I detailed the closer tentacle with some 3dish looking soft brush strokes and a texture on overlay. I wanted to make sure the viewer felt a little grossed out. To give the viewer an uneasy feel perspective-wise, I emphasized the dutch angle by adding vague mountains in the background that pointed up and to the left and put the diver's boat in view. If the implied horizon and boat work properly, the viewer should feel off balance (i say if because my wife didn't notice the boat).
All done in Photoshop with my Intuos4!