These are terrestrial worlds covered in a world spanning ocean tens, or even hundreds of kilometres deep. They have no surface land, and will often have a thick atmosphere that can be high in Oxygen.
Such worlds can harbour life, and if so will have a variety of sea life, though not as varied as more traditional terrestrial worlds. With no continents, there are no shallows where land and sea can mix. The ocean floor is often too deep for life to survive, and is often covered in thick layers of super-dense variety of ice known as Ice VII, meaning rock and water don't mix.