Laythe

Laythe is an Ocean moon orbiting Jool. It is the closest moon to Jool, and the second biggest moon in the Kerbol system after Tylo. It is also the only moon with an atmosphere in the Kerbol system. Though in the position of Io, its closest real life counterpart is Titan, due to its atmosphere and bodies of liquid. In ksp 1, it's atmosphere is breathable by kerbals, which could presumably mean it is also the same in ksp 2. If so, it could be a good place for colonization.