Ocean tiles may only be placed on areas reserved for ocean (see map). Placing an ocean tile increases your Terraform Rating (TR) 1 step. Ocean tiles are not owned by any player. Each ocean tile on the board provides a 2M€ placement bonus for any player later placing a tile, even another ocean, next to it.
Each ocean tile represents 1% ocean coverage. With 9% of the surface covered by oceans, Mars will have a hydrological cycle, creating rain and rivers. This symbol means that you place an ocean tile, and therefore also increase your TR 1 step (and get any placement bonus).