GeoTIFF Shoreline Alpha Fade Task

I need a QGIS or GIS-skilled freelancer to add a correct alpha transparency gradient to a single GeoTIFF orthophoto tile (provided).

The goal is to preserve the land and beach fully opaque, keep the bright turquoise shallow-water zone (≈150 m offshore) mostly opaque, and then create a smooth fade to full transparency starting about 150–200 m from the coast, so the orthophoto blends naturally with X-Plane’s simulator’s default ocean.

Requirements
• Maintain original georeferencing, resolution, and extent
• Export GeoTIFF with ALPHA=YES (embedded alpha channel)
• Provide a short confirmation screenshot from QGIS showing partial transparency in the alpha band

Deliverable
• One GeoTIFF with proper alpha fade beyond shoreline

If successful, the workflow will be extended to the full Mykonos coastline tiles (3 more tiles or similar size)