It's a bunch of factors combined. The terrain is part of it, but the elevation the terrain sits at is also a huge factor. Other factors include latitude, and also neighboring terrain features like how far to the ocean, where the mountains are, the nature and type of prevailing wind patterns. You can possibly build up an artificial mountain, but not a mountain range, and you're not moving oceans around.

Likely the best you can do is pick a perfect spot for an island and build it. Or you can dig canals and mess around with water flow. You have places like the Salton Sea and the current condition of the Aral Sea which are results of man-made water infrastructure.