Originally Posted by chucklesR
If you can see the people in the cockpit, use their height to estimate 6 ft, which establishes a scale from which you can estimate within 2-3 feet.
That's essentially what I do for boat's under way. Can't always use a person's height as a sale of measure, unless crew is standing upright on deck. Head to waist for normal adults is about 2.5 ft, 3.5 ft for deck to waist. Or - about 7 adult heads equals 6 ft . . . 5 heads for narcissistic sailors
After a few years of studying various boats and their known freeboards, you get a knack for it, with or without people nearby.