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.

From space, shallow-oceaned planets look very similar to Earth-like planets, the oceans appearing just slightly smaller. Since it is the surface of the oceans and not their depths that generates water vapor, clouds and rain, percipitation is very similar to that on Earth-like planets. The shallow oceans is a much poorer temperature buffer however, so temperature changes will be much greater than on Earth-like planets (not as severe as on desert planets though). The effects of this on habitability varies depending on many factors such as greenhouse effect and orbit. If a shallow-oceaned planet is also a low - spinning planet and/or elliptical orbit planet, that is not good.

Unlike Earth, where much of the ocean is "marine desert" with the sunlit surface layers very poor in nutrients, shallow-oceaned planets would be able to support rich marine life in almost all of their oceans with sunlight reaching most of the ocean floor and silt being much less diluted. If a shallow-oceaned planet has native life the vast shallows probably helped the transition from ocean life to land life. The rich ecosystems in the oceans would allow huge ocean animals to find enough food without long migrations, so blue whale sized beings could exist despite their obvious problems with migrating across shallows.

In the absence of significant amounts of water pressed into the mantle, the tectonics would be somewhat different from Earth-like planets and produce exotic geological features.

Early Mars was probably a shallow-oceaned planet billions of years ago.

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