| Sg. Indef. | vatnstankur |
| Sg. Def. | vatnstankurinn |
| Pl. Indef. | vatnstankar |
| Pl. Def. | vatnstankarnir |
| Sg. Indef. | vatnstanks |
| Sg. Def. | vatnstanksins |
| Pl. Indef. | vatnstanka |
| Pl. Def. | vatnstankanna |
| Sg. Indef. | vatnstanki |
| Sg. Def. | vatnstankinum; vatnstanknum |
| Pl. Indef. | vatnstönkum |
| Pl. Def. | vatnstönkunum |
| Sg. Indef. | vatnstank |
| Sg. Def. | vatnstankinn |
| Pl. Indef. | vatnstanka |
| Pl. Def. | vatnstankana |