More accurate (but more difficult) is to use two scales. Balance the boat on two scales set on a hard surface. Mast down.

The scales' error is additive. If each scale's accuracy is +/- 1 lb, that means 4 scales = +/- 4 lbs; 2 scales = +/- 2 lbs. Just because the display reads to the 0.1 lb doesn't mean it's accurate to that amount. Calibrate the scales with a known weight that's close to the expected weight of the thing you're weighing. (For an F-18, 2 scales = 200 lbs each).

Load cells are still the best (most accurate) way to weigh boats.