re= float- measurement of liquid.
The whole point is, the total volume is measured. It is a given that a certain amount of tank-contents is UNAVAILABLE TO THE PICKUP -This allows for dirt, water and other contaminants to settle clear of the drawoff point.....a well-designed tank would have a low-point sump well away from the pickup and incorporating a transparent sedimenter and drain point. there is absolutely no point in drawing a sample from a higher level,which is why some designs need multiple drain-points.
Having said that, PROVIDED regular sample- drains are carried out, the water coalescing ,is pretty-much irelevant.....IT IS IN THE UNUSABLE "SUMP" AREA
The CONTENTS gauge will show how much usable fuel you have......if a portion of that is water, then you, the Pilot, are guilty of gross incompetence and negligence.....retire immediately!
A Capacitative system, calibrated to a laboratory-standard,pure liquid, is not fit for the purpose of measuring a liquid of "stated" quality. Clearly, there's a tolerance on commercial fuel hudrocarbons...I guess the on-board computers and sensors are not smart -enough to pick up a deviance caused by contaminated fuel....which brings us back to my original proposition....
An electronic system of fuel tank contents measurement is not fit for commercial aircraft operations....it's measurement is arbitrary and dependant on the quality of the fuel.
A sump-drain will not necessarily show water, but that does not mean there is not a high water content dispersed in the fuel itself (see the Heathrow BA crash threads for that one! ) Ultimately, it can precipitate, but you need a very large amount before you have trouble under normal operating and check regimes.
Helicopters have a poorer unpowered flying ability than fixed-wing...therefore it would seem a priority to design any such craft around an "Inherently safe" regime. Eliminate as many traps, "gotchas" and flaws at the design stage...together with good ergonomics, a safer aircraft should ensue.