The really, really big waterwords can look like gas giants at first glance; the giveaway is the complete lack of stripes (although the icecaps can look like stripes at first glance too). The biggest I've personally found is about thirty earth-masses.
These worlds are never terraformable, of course (anything above surface gravity 2.0 is invalid, which typically means a mass of above 4.0 to 4.2, depending on planet density). So they may be big and cool-looking, but they're not the most valuable waterworlds out there.