Strip of light, laser projection or whatever

Try some of the marking paint I mentioned. It's what they use to mark the ground where people will be digging and need to know where utilities are buried. It sprays upside down. The paint is biodegradable and will wash away from a few rains. Spray it next to the drivers side tracks next time you pull the boat out....just to the house edge of the track, so you can use it as a guide when you back in upon return. Keep your trailer tire right by the paint and you should have an easier time.