Nitrogen Seas are essentially what they sound like. Cold, icy worlds with a massive ocean buried beneath a solid layer of ice. An ocean made of liquid nitrogen. They have an unbreathable atmosphere (probably pure nitrogen gas), so you'll want oxygen on your person as well as some sort of way to protect against the frigid air.
Codex description Edit
Doesn't this sound like a fun place to swim? Yea, it really isn't. Some handy liquid there, though.