Soundings: In olden days, the leadsman throws lead overboard at the right moment in front of the boat, and as the boat moves forward the lead sinks rapidly. At the moment the boat overruns the lead, it tis straight up, and the leadsman shouts out the depth. Two people with sextants watch the angles change and get the angle to the two marks. Knowing those two angles defines a point. Once the point is defined, you mark down the sounding.
You have to record the time the sounding is taken, so the soundings can all be converted to the same reference datum. You need to know what the tide is!

Tide reference datum: "Standard elevation defined by a certain phase of the tide." (NOAA)
Semi-daily tide (aka semi-diurnal): Two low tides and two high tides of approximately similar heights over the period of one day.

mean lower low water: the average height of the lowest low tides

(On class chart)
Contour intervals - the spaces between lines on topographic maps
Look @ Mt. Bowditch. See how the mountain goes up by 100 ft every line? That is the contour interval.

Storm surge can make the normal high tides and low tides much higher than usual, and can cause a lot of damage.