Doodled with my finger using Autodesk Sketchbook Express for mobile devices. They recently improved the brush quality, and the program is sooo much better now. If you have an iPad/iPhone/iPod touch and want to use it for digital art, definitely give this app a try - it’s free, too!
I might do a non-doodle, higher quality version of this general idea later. Mermaids are pretty awesome.