I'm a bit tired, so it's difficult for me to figure out the exact thing you're trying to do and exactly what to do to achieve it, but I think the tools you are missing are the "paste in place" command (under the Edit menu, or Ctrl + Alt + V) and boolean functions, specifically Difference and Intersection, under the Path menu. I'm pretty sure I've explained this to people before so I should have some images I made to demonstrate it to them, let me check.
Okay these'll work.
Okay, you got your sea and you got your land shape. You want to make a country shape with its boundaries being said country's land borders and coastline. What do? Well, draw the country's land borders and then connect them up around the coast, so that the overlap between the country shape and the land shape is the exact shape of the country, like this:
What you do now is copy the land shape and press Ctrl + Alt + V to paste it in the exact same position as the original. Now select that land-shape-copy and your country shape, then go to the Path menu and select "Intersection". This will remove the two shapes and give you a shape of the overlap between them, like this:
Ta-da! A country shape whose boundaries are said country's land borders and coastline. One issue you might have now is that the country-shape is on top of the land-shape, so the country's borders will show up over the coastline, like how the red line is showing up over the black coastline in that image. In order to solve that I copy the land-shape, paste it in place on top of everything, and then remove the fill, so you have a shape that's just the coastline on top of everything else.
But yeah, hopefully that helps. The paste in place function and the boolean functions are really useful and versatile - I use them a loooot when making anything in Inkscape - so I'd advise you to play around with them a bit to see how they work and what they do. And if you need any further help, just ask.