For that, you'd want to find an artist whos style you like. Specifically, look for someone who's done ocean scapes, which can be hard and if they're good it'll be a pretty penny. But, if you're like me, you'd think quality > $$$. For something like this, even if you have someone draw something up ahead of time, it'll be hard to find a tattoo artist that can bring it to life. Tattooing is very very different from drawing or painting and it's hard to do most styles. You could find a couple references, for example, a background that you like, and a picture of a seapony that you want similar to, and then give them a detailed explanation of what you really want. Most good tattoo artists will not just copy some art work. They create something new and unique that is one of a kind for your skin.