Hey swiss, this is the way I usually do it.
Step 1: Make a selection of the Land layer (cmd/ctrl+left click layer thumbnail)
Schermafbeelding 2017-11-21 om 17.29.39.png
Step 2: Select > Modify > Expand... For this example I filled it 10px, but you can fill in any number you like.
Schermafbeelding 2017-11-21 om 17.29.47.png
Step 3: Select > Modify > Smooth... Again used 10 for this example, but you can use whatever you need to get rid of the angular corners of the selection.
Schermafbeelding 2017-11-21 om 17.29.58.png
Step 4: Edit > Stroke...
Schermafbeelding 2017-11-21 om 17.30.08.png
Step 5: choose how think you want the stroke to be. I set it to 2px, 100% opacity.
Schermafbeelding 2017-11-21 om 17.30.27.png
Step 6: Now you have the stroke added. if you want more, just repeat the past steps until you're satisfied.
Schermafbeelding 2017-11-21 om 17.32.55.png
I hope this is any help (: