Another frequent question of my students is how to obtain a polygon map of the seas and oceans, rather than the land polygons (countries, etc.) that are commonly imported with R spatial data packages. You can mostly just use the land polygons and do the opposite operation as you would do for terrestrial features — e.g., to colour the sea in a map of land polygons, just use the background instead of the col argument; to mask a raster map to the sea, just use the land polygons for a terra::mask() with inverse=TRUE; to select or crop features that overlap the sea, just do instead terra::erase() with the land polygons. But anyway, you may just prefer or need to have a marine polygon.

There are marine polygon maps available for download, e.g. at Marineregions.org. But if you don’t need the separation into particular seas or oceans or EEZs, you can easily create a global marine polygon from a countries map in R:

# import a world countries map:
countries <- geodata::world(path = tempdir())
terra::plot(countries, col = "tan")

# make a polygon map delimiting the entire extent of the Earth:
earth <- terra::vect(terra::ext(), crs = "EPSG:4326")
terra::plot(earth, col = "lightblue")
terra::plot(countries, col = "tan", add = TRUE)

# erase the countries (land parts) to get just the marine polygon:
marine <- terra::erase(earth, countries)
terra::plot(marine, col = "lightblue")

That’s it! See also terra::symdif(), or terra::mask(inverse=TRUE). You can then crop the marine polygon with your own other polygon or desired extent, e.g. terra::crop(marine, terra::ext(-20, 60, -40, 40)); and/or you can use the marine polygon to crop/mask other maps to the marine regions, e.g.:

# import a global bathymetry map:
bathy_source <- "/vsicurl/https://gebco2023.s3.valeria.science/gebco_2023_land_cog.tif" # from https://gist.github.com/mdsumner/aaa6f1d2c1ed107fbdd7e83f509a7cf3
bathy <- terra::rast(bathy_source)
# terra::plot(bathy)  # slow

# crop bathymetry to a given extent:
bathy_crop <- terra::crop(bathy, terra::ext(110, 180, -50, 0))
terra::plot(bathy_crop, main = "Pixel values everywhere")
terra::plot(countries, add = TRUE)

# crop and mask bathymetry to keep values only on the marine polygon:
bathy_marine <- terra::crop(bathy_crop, marine, mask = TRUE)
terra::plot(bathy_marine, main = "Marine pixel values only")

See also this previous post for how to further crop/mask to the near-shore raster values, including for particular continents or islands.