One of my favorite geographic features I've ever seen in a fantasy world is a vast, shallow sea in Roshar, the world of the Stormlight Archive series. It's massive, but less than a meter deep, so you can comfortably wade through it for miles in any direction.
@Felthry He has, actually!
Book 3 comes out in November. :-)
https://www.goodreads.com/book/show/17250961-oathbringer