Yes, coming from a "scientist", water immersion is the most accurate (until we invent some of those cool Star Trekkie gadgets - although we already have some of them
)
Still, the other gadgets, if used by a trained professional, do a pretty good job of giving you a "ballpark" idea.
In MOHO its better than the BMI, which I think is good for the "average" person.
This is what is shown at the bottom of the BMI calculator on the TSFL site:
Please note: There are some limitations to the Body Mass index calculation, because it does not measure body composition. BMI overestimates body fat in persons who are very muscular and it can underestimate body fat in persons who have lost muscle mass (e.g. many elderly). People with more muscular builds may be classified as overweight according to BMI numbers even if they have a very healthy body fat percentage. A high BMI from increased muscle alone is not associated with health risks.