| Sg | Pl | |
| Nom | vatn | vötn (rule) |
| Acc | vatn (rule) | vötn (rule) |
| Dat | vatni (rule) | vötnum (rule) |
| Gen | vatns (rule) | vatna (rule) |
| Sg | Pl | |
| Nom | vatnit (rule) | vötnin (rule) |
| Acc | vatnit (rule) | vötnin (rule) |
| Dat | vatninu (rule) | vötnunum (rule) |
| Gen | vatnsins (rule) | vatnanna (rule) |