Shallow-oceaned planets are intermediate between Earth - like planets and desert planets. On Earth-like planets, intense water pressure at the bottom of deep oceans press much of the water into the mantle, Earth has about 20 times more water in its mantle than in its oceans. Since water spreads out, the primary effect of reduced water volume is reduced ocean depth rather than reduced ocean surface. So worlds that get a bit more water than desert planets, but much closer to desert planets than Earth-like planets, get shallow oceans that cover about half the surface.