Abstract: A guide device for attachment adjacent the rear of a boat trailer to assist the driver of a towing vehicle in backing the unloaded trailer into the water. The guide device includes a mounting bracket for attaching the device onto a rear part of the boat, such as onto the front face of the rear lateral frame member, and a signal member mounted on the bracket and pivotal between raised and lowered positions. The signal member includes an elongated staff above the bracket and a flotation element attached to the lower end of the staff beneath the bracket. The flotation element is lighter than water, and has a weight greater than the weight of said staff. When the trailer is out of the water, the staff is pivoted by the weight of the flotation element to a generally vertical position. When the rear of the trailer is backed into the water, the flotation element rises to pivot the staff to a generally horizontal position.