You've got the right idea but the wrong tool.
I love my Nikon range finder, works great day or night up to 600 yards. I sleep better at night knowing exactly, down to the yard or meter how much swing room I have.

You can generally find them on sale for $100. Of course you can spend a lot more for further range and higher accuracy, and for golfing or shooting that may make sense. But a basic one is all you need for boating.