I found a place to measure the water level on the small lake I'm mapping.
There is a concrete outlet pipe structure that is currently above water.
I think it was under water at the highest level last spring. So I can measure
the level below or above this structure. Based on the manual, I believe I can
enter the level below the structure as negative and the level above the
structure as positive. Is that correct?

The Track Edit Pane

(3) Water Level

The overall depth of the track can be adjusted up or down by using the
slider, or entering a value in the text box. The water level is adjusted
by creating a single tide point, which is then applied to all points
in the track. Note that this function is disabled if tide values have
already been set manually.

If you enter a correction as - you make your loggfile deeper. (Ie you do this when you have logged at a low waterlevel). If you enter your correction as + you make your logfile shallower (ie you do this when you have logged at a high waterlevel).