You should add Enceladus. It is a moon of Saturn that is very small but has massive geysers at it's south pole that are connected to an underground ocean the size of lake superior. Though is is made of ice and is very similar to europa, I think it could work well. Maybe have a way to travel across the moon to the geysers were you could like jump in or dig down into the underground ocean where you could find cool creatures, and yes, a boss. NOTE: The ocean would be finite, so it would have an ending, and would be fairly deep under the southern ice.