>you quote was intended to be 29-31 inches Hg rather than mm Hg, then you
obviously

Yup, I human'd again.  Should be inches.

>have a free surface near sea level.  If the 29-31 mm is correct, then you are
either

Adequately near.  The zero offset is easily obtained as part of the startup
code.

>substitute the density of water for the density of air you used in your earlier
>airspeed project.  If you are talking about boat speed rather than water speed,
then

Earlier project didn't give absolute airspeed, it just kept the plane flying
within a small window of perceived airspeeds (I forget the correct term, but
basically it means the difference in pressures between ambient and pressure
(speed) pickups).

For this one I need an absolute speed out.

>you don't need to make a total depth adjustment.

I expect a typical dept to be 1/8" submerged when at speed; at this depth, I
doubt I'll even know that there is a difference with the sensor.

Thanks for clarifying those items.

I guess the better way to word the question is, "How much pressure does water
exert on a body moving forward?"

Andy