The datasheet Eric linked to had, pressure, density @ temperature and G.
I THINK if one were to look at relative height, you would need a ported sensor or one related to differential pressure. So, i would think you would need calm waters.
Acceleration due to gravity varies. See:
http://www.blazelabs.com/f-u-massvariation.asp. Density will vary with temperature. If then sensor has any smart, the temperature would be measured at the sensor. The temperature may not be the same over the measured distance.
Further remember that the pressure REPORTED by the weather bureau is NORMALIZED to sea level even if your 5000 ft up.
You have a lot of potential error sources because this isn't a tank on the ground.
Now, I'm wondering if you could do a differential sort of measurement to a known depth (floating sensor).
Say the floating sensor is 1 foot under water, but it reports a value of 13". Your sensor reports 26' The density of the water may change with temperature and depth.
So, you sort of normalize it.
These are just ideas.I haven't worked in the field. I have had hydraulics and relativity in class, but don't remember much.