[cripplecreek]

“Mars for example is in the sun’s habitable zone, but it has no oceans – causing air temperatures to swing over a range of 100°C...."

Actually that temperature swing is mostly the result of a very thin atmosphere. Oceans do help hold and transport heat around a planet but they're hardly the only mechanism that makes a planet warm.

[ArtDodger]

And how do we know there isn’t a sizable ocean of water under the surface?