I would say roll Arcana, since Talogan won't know, but if you ask Nyx she'll mention that water wouldn't affect her Witch Bolt in any meaningful way. It wouldn't electrocute anyone beyond her original target, not to mention she'd have to be within 30 ft to hit them anyways. She would explain though that Lightning Spells cut through water more easily than fire or cold spells; fire spells not only have difficulty getting through water (made at dis) but they also lose out on damage (Being fully submerged in water gives you resistance to fire). On the other hand various forms of cold damage such as her ray of frost would have a bit of difficulty reaching the target (attack at dis) due to how how the water would constantly eat away at the energy she puts into a beam, but the damage will remain the same. Lightning spells have neither issues as long as Nyx can see the target, and would just have to roll her attacks and damage as normal for the spell.