In hydrographic surveying and navigation, identify the correct statements: (a) for nautical purposes, navigators generally refer depths to the low-water level; (b) the measured depth from the water surface to the sea bed is termed a sounding; (c) contour lines drawn on the bed of a water body are represented by fathom contours.

Introduction / Context:
Hydrographic surveying provides the foundational data for safe navigation and coastal engineering. Three fundamental notions are the vertical datum used by mariners, the term used for measured depth, and how underwater relief is depicted on charts using contour lines expressed in fathoms.

Given Data / Assumptions:

• Standard hydrographic practice uses a low-water datum for conservative clearance.
• Depth measurements are taken from the instantaneous water surface to the bed.
• Older or traditional charts often express depth contours in fathoms (1 fathom = 6 feet), termed fathom contours.

Concept / Approach:
Choosing a low-water reference reduces risk because charted depths are least at low tide. A measured depth is a 'sounding'. To visualize submarine topography, contours join equal depths; when depths are noted in fathoms, these are referred to as fathom contours.

Step-by-Step Solution:
Confirm (a): Navigational charts commonly adopt a low-water datum (e.g., Lowest Astronomical Tide) so mariners can rely on minimum clearances.Confirm (b): A sounding is the measured vertical distance from the water surface to the bed.Confirm (c): Underwater contour lines are depth contours; when labelled in fathoms they are colloquially called 'fathom contours' (often shortened to 'fathoms').Therefore, all three statements are correct.

Verification / Alternative check:
Compare against standard hydrographic terms: datum at low water, depth = sounding, and contours by equal depth values.

Why Other Options Are Wrong:

• Choosing only one statement ignores the integrated standard practice reflected in all three.

Common Pitfalls:

• Confusing chart datum (a reference plane) with tidal levels at a specific time.
• Assuming 'fathoms' means random depth points rather than contour lines connecting equal fathom values.

Final Answer:
All the above.