Credited to u/darkblade273.

The most common map guides I’ve found glance over coastlines as just “use a fractal generator” or “just make it really jagged and never use straight/curved lines”, which are only really suitable for some regions of the world. Coastlines are affected by elevation, underlying tectonic plates, and climate, which are never really touched upon, so I made this short guide as to different types of coastlines.

Here are pics of real world coastlines of varying sizes with parts outlined in black, to better help visualize what coastlines look like at various levels of zoom in.

Any Elevation, Temperate / Warm / Hot Regions

• The most common coastal pattern, straight lines that sometimes bend into points or have very small inlets/estuaries for rivers or tiny islands at the coast
• No recent glacial activity, so no fjords or inlets like in glacier affected regions

High Elevation, North/South Regions

• Glaciers carve fjords and islands into the elevated areas of the coastline
• Lots of tiny little bays and inlets, also lakes
• In other areas coastlines are rigid, but fairly straight

Low Elevation, North/South Regions

• Glacier activity still carvves lakes and islands, but not as dramatic and no real fjords
• Mostly straight lines that curve or come to points at places
• Lots of lakes still

Oceanic Island Chains

• Oceanic island chains are where Oceanic plates converge with other places, they result in chains of many volcanic islands following the boundary of the plate
• Glacial activity is possible if islands are close to poles, like in eastern Russia, which results in fjords and islands
• Even without glacial activity, have large amounts of tiny islands surrounding big islands, all of them volcanic

Continental-Continental Mountainous Regions*

• * In areas where continental plates converge with an open sea coast, like the Mediterranean Sea or the Persian Gulf, there’s an increase of islands around the main peninsulas and coastlines
• Lots of islands with jagged coasts and inlets, closest to the typical fractal generation