An improved boat trailer guide-on comprising bracket means that adjustably attaches to a conventional boat trailer and holds a substantially vertical shaft means with telescopically extendable foamed plastic rollers thereon. Such a device allows the roller to float upwards as the boat trailer is backed into the water, thus remaining visible and capable of assisting in the loading and unloading of a boat. Optionally, the top of the floating, telescopically extendable, roller is equipped with a tail light lamp for assisting in loading and unloading after dark.