Grow. Hunt. Survive the abyss.
Steer with your mouse or finger. Hold down to dash — faster, but you grow slower while it's held, and you're moving too fast to react as well. Small sea pearls, pink shells and darting fish are common food — eat them to grow. Gold pearls, coral starfish and scuttling crabs are rare and give a real growth spike. Spiky red pufferfish shrink you — avoid them, and don't graze too close, they'll pull you in. Grow noticeably bigger than another creature and you can eat it on contact — get too close to something bigger and it eats you the same way. Close to the same size? Whoever's head hits the other's body goes dark. Other creatures will hunt anything smaller than them, so don't dawdle. Stray past the reef light into the abyss and you go dark too.
Every other snake is down. You're the last one swimming.