Dark moves are useful against ghost-types. Liquid knows Bite and I am sure he'll learn some new moves once he levels up in the future. I've not actually seen any ice moves in action, so it should be special to watch for you.

[Otacon thinks they're special anyway.]

Piplup might be able to learn ice moves once he levels up, since he's a water-type too. [He noticed the little penguin nodding and smiling.]