The water portion of Victory Road is first. There may be parts where you must dive, parts you must use waterfall, so make sure you have Pokemon for the journey to the Elite Four building who know these. You do not need to fight with these Pokemon in the Elite Four. All the water types here are strong, so make sure you have a strong grass or electric type, or anything that can finish them off. Maybe you'll catch one, and maybe even use it in your upcoming battles!