In offshore wind turbine foundation construction, the seabed settlement induced by pile driving and structural self-weight occurs concurrently with local scour driven by tidal currents, directly affecting foundation stability and backfill design. However, few case studies have quantitatively evaluated construction-stage bed-level changes and verified placed backfill volumes using sensor-derived three-dimensional bathymetric data. In this study, we propose an uncertainty-aware procedure for analyzing seabed settlement and scour and estimating backfill volume at an offshore wind monopile foundation using two-epoch XYZ bathymetric datasets acquired by a USV-mounted multibeam echosounder (MBES) integrated with positioning, attitude, sound velocity, and tidal correction sensors. The two datasets represent conditions after pile driving but before backfilling and after the completion of backfill placement. Grid-based elevation differences (ΔZ) were computed after registering both datasets to a common coordinate system and a 0.25 m grid. A reference seabed surface was introduced to decompose volumetric change into required backfill, actual backfill, and overfill components. Cross-line and reference-zone residuals were also used to define a depth-of-difference uncertainty threshold, preventing changes within measurement uncertainty from being overinterpreted. The case study showed that the most pronounced bed-level changes were concentrated near the monopile. The estimated actual backfill volume was approximately 178.7 m³, whereas the net volumetric change over the analysis domain was approximately 169.3 m³. These results demonstrate that MBES-based acoustic sensing and uncertainty-aware XYZ data analysis can support rational backfill design and post-construction performance verification for offshore wind turbine foundations.