The best replacement for coastline is shoreline.

The best alternatives to coastline are rated as following:
★★★★★  shoreline
★★★  coast line
★★★  coastline
  tidal shoreline

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Speaking about coastline, one probably shouldn't forget about atlantic ocean.

Other search terms related to coastline and their ratings are:
★★★★★ atlantic ocean
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coastline
shoreline
coast line