Chart depths are mean lower low water (MLLW), which is a bit lower than mean low water (MLW).
Bridge heights, etc... from Bowditch:
The shoreline shown on charts is generally mean high
water. A light’s height is usually reckoned from mean sea
level. The heights of overhanging obstructions (bridges,
power cables, etc.) are usually reckoned from mean high
water. A high water reference gives the mariner the minimum
clearance expected. Since heights are usually reckoned from high water
and depths from some form of low water, the reference levels
are seldom the same. Except where the range of tide is
very large, this is of little practical significance.