Normal Water Runes need a Oceanstone along with a Blank Rune; a Silver Ingot and three Weak Water Runes to make one Normal Water Rune.
Normal Runes can be crafted at level ten at the Rune Altar.
You need three of these to learn Water Slash and you will need extra to cast the spell.
The command to make a Normal Water Rune is [nya!rpg rune altar water]. For more info use [nya!info normal water rune].