A safety device automatically raises and lowers a warning flag over a boat based on the tension applied to a line towed by the boat. For the device, the tow line is anchored to a tower mounted on the boat. Further, an arm is pivotably connected to the tower for movement between first and second orientations. Structurally, the arm is partially enclosed within a housing mounted to the tower. Also, the warning flag is pivotably mounted to the housing. Within the housing, the device includes a mechanism for translating movement of the arm to movement of the flag between raised and lowered orientations. When the tow line is slack, the arm is biased toward the first orientation by a spring. However, when the line becomes taut, the tension in the line overcomes the spring force and the arm is moved to its second orientation.