GeezerVison

import matplotlib.pyplot as plt
import cartopy.crs as ccrs
import cartopy.feature as cfeature

main_map_proj = ccrs.TransverseMercator()
pcarree = ccrs.PlateCarree()

fig, ax = plt.subplots(subplot_kw={"projection": main_map_proj})

ax.set_extent([-50, 15, 50, 70], crs=pcarree)
ax.add_feature(cfeature.OCEAN.with_scale('50m'), facecolor="lightsteelblue", zorder=1,
               edgecolor="k", lw=0.2)

inset_ax = ax.inset_axes((0.05, 0.05, 0.3, 0.3), projection=pcarree)

inset_extent = [-44, -38, 63, 66]
inset_ax.set_extent(inset_extent, crs=pcarree)
inset_ax.add_feature(cfeature.OCEAN, facecolor="lightsteelblue", zorder=1)

ax.indicate_inset_zoom(inset_ax, edgecolor="black", transform=pcarree)

x = [inset_extent[0], inset_extent[1], inset_extent[1], inset_extent[0]]
y = [inset_extent[2], inset_extent[2], inset_extent[3], inset_extent[3]]
for a in ax, inset_ax:
    a.scatter(x, y, c=range(4), s=150, transform=pcarree)

plt.show()