Hi,
If i understand, you want to built a mesh finer in the coastal and large in the offshore.
So like, what suggested you Coulet, you should use the density area.
With blukenue, you create lets say 2 density area, so you create two closed line, make sure to impose a value for your closed line in m, this value will be the resolution of your area. Make this for the two closed line created by bluekenue, after this gliss him in the density area and built your mesh.
Hope that help you.
REGARDS.
YASMINA.