First off, you don't use a fishing pole, rather, you're an odd little orb that shoots oxygen out to kill oddly shaped fish. Your goal is to kill them in a way they remain intact, then cash them in for upgrades. WASD controls your movement and your mouse will help you aim. Along the way, you'll also come across notes left by captains and when you sleep for the evening, you'll occasionally have odd dreams.
So, you play as an orb shooting oxygen out to kill fish and you occasionally dream about evil fish when you complete the day -- it's a rather odd experience, but it's well worth checking out for yourself, especially when you get to the strange endgame. It's a simple experience and there isn't much to say about it, but it's still a worthwhile way to spend a few minutes of your time.