This paper proposes a novel concept of distributed sea surveillance system which consists of an active leader boat and multiple passive tethered follower boats for distributed sensing on the sea. The active leader pulls the follower boats and the trajectory of each follower is controlled by a rudder attached to it. The passive follower boats are energy efficient and lightweight, and many follower boats can be deployed simultaneously. By tethering many passive boats, we can make a formation and cover a large area for the surveillance. The followers keep the formation using a decentralized control algorithm. In this paper, we propose a formation control method with obstacle avoidance function for multiple passive follower boats and the simulation result illustrates the validity of the proposed control method. We also develop passive follower boats and conduct experiments for realizing several formations of two passive boats by using the flow of the river.