Water Runes need a Oceanstone along with a Blank Rune; a Silver Ingot and three Weak Water Runes to make one Water Rune.
Normal Runes can be crafted at Arcane Forging 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 Water Rune is [@Neko Chan#7837 rpg rune altar water]. For more info use [@Neko Chan#7837 item info water rune].