memory has just kicked in...
Places along rivers used to use special wierdo gauges.
They were just like a small floating device in a tube.
the top was open, but only enough to let air in and out, not the floating device.
no the tube goes to the bottom.
the floating device is only just big enough to fit inside the tube.
and this way, it removes most of the sloshing and gives a fairly accurate reading.
this is what they used to use at some places along rivers to get tide height, (Like on river thames) and the waves dont give an innacurate reading.